According to the ACAMS Study Guide 6th Edition, Chapter 2, page 38, one of the steps that a financial institution should take when it suspects money laundering activity is to file a suspicious transaction report (STR) or a suspicious activity report (SAR) to the relevant authorities. This will alert the regulators and law enforcement of the potential money laundering scheme and provide them with valuable information to trace the funds and identify the perpetrators.
Another step that a financial institution should take is to identify any unusual or suspicious wire transfers initiated by casas de cambio to jurisdictions outside of Mexico that bear no apparent business relationship with that casa de cambio. This could indicate that the casas de cambio are involved in layering and integration stages of money laundering, where they are moving the illicit funds across borders and disguising their origin and ownership. The financial institution should monitor and document these wire transfers and report them to the authorities if necessary.
Option B is not a relevant step to trace funds through the bank, as it does not provide any information about the source, destination, or purpose of the funds. A decrease in the sale of large denomination U.S. bank notes to casas de cambio by the bank could be due to various factors, such as market demand, exchange rates, or regulatory changes, and does not necessarily indicate money laundering activity.
Option C is also not a relevant step to trace funds through the bank, as it does not indicate any connection to the suspected money laundering scheme. Deposits by casas de cambio that include third-party items, such as sequentially numbered monetary instruments, could be legitimate transactions that are part of the normal business operations of the casas de cambio. Unless there is evidence that these deposits are related to the sale of large denomination U.S. bank notes or the Mexican syndicate, they are not useful for tracing the funds.
References:
ACAMS Study Guide 6th Edition, Chapter 2, page 38
Combating Money Laundering and Other Forms of Illicit Finance
Sting Operation Snares Casa de Cambio for Allegedly Laundering Millions Through U.S. Banks

A suspicious transaction report (STR) is a document that financial institutions must file with the relevant authorities when they detect or suspect any activity that may be related to money laundering, terrorist financing, or other criminal offences. An STR should include the details of the customer, the transaction, the reason for suspicion, and any other relevant information.
One of the indicators of suspicious activity is the use of financial instruments, such as checks, money orders, or cashier's checks, that are drawn on other institutions and deposited in amounts under the currency reporting thresholds. This may suggest that the customer is trying to avoid the detection and reporting of large cash transactions, which is a technique known as structuring or smurfing. Structuring is illegal and may indicate that the customer is laundering money from illicit sources or evading taxes.
Another indicator of suspicious activity is the use of financial instruments that are sequentially numbered, meaning that they have consecutive serial numbers. This may suggest that the customer has obtained the instruments from the same source or issuer, and that they are using them to launder money or finance illegal activities. Sequentially numbered instruments may also indicate that the customer is involved in fraud, such as check kiting or counterfeit checks.
Therefore, if a bank conducts a quarterly review on a retail customer's account and finds that the customer has made a number of large deposits of financial instruments drawn on other institutions in amounts under the thresholds, and that the instruments were sequentially numbered, the bank will most likely file an STR. This is because this activity does not fit the customer's historical profile, and it raises a high level of suspicion that the customer is engaged in money laundering or other criminal activities.
References:
CAMS Study Guide - 6th Edition, Chapter 4, Section 4.2, page 108
Answers to Frequently Asked Questions Regarding Suspicious Activity Reporting and Other Anti-Money Laundering Considerations, Question 5, page 7 Anti-money laundering - handling suspicious activity, Section 6.3, page 3 Tell us about suspicious activity that may be linked to money laundering, Section 3, page 1

A bank's risk assessment is a process of identifying, measuring, and mitigating the potential risks that the bank faces in its operations, products, services, and customers. According to the ACAMS Study Guide, a bank should focus on the following areas when performing a risk assessment1:
The type and location of the institution's clients. This involves analyzing the customer base, the types of accounts and transactions, the source and destination of funds, the level of due diligence and verification, and the risk profile of the customers. For example, a bank should consider whether its customers are individuals or entities, domestic or foreign, politically exposed persons, high-net-worth individuals, non-profit organizations, or cash-intensive businesses. The location of the customers may also indicate the level of exposure to money laundering, terrorist financing, sanctions, or tax evasion risks.
The nature and breadth of the services and products the institution provides. This involves evaluating the range and complexity of the products and services offered by the bank, the delivery channels, the payment methods, and the innovation and technology involved. For example, a bank should consider whether it offers wire transfers, correspondent banking, trade finance, private banking, trust and fiduciary services, prepaid cards, mobile banking, or cryptocurrency services. The nature and breadth of the services and products may also affect the level of transparency, traceability, and compliance of the transactions.
The geographic locations where the institution does business. This involves assessing the jurisdictions where the bank operates, where its customers reside, where its counterparties are located, and where the funds flow. For example, a bank should consider whether it has branches, subsidiaries, or affiliates in high-risk countries, whether it serves customers from high-risk countries, whether it engages in cross-border transactions, and whether it complies with the local laws and regulations of the countries where it does business. The geographic locations where the institution does business may also influence the level of exposure to political, legal, regulatory, or reputational risks.
References:
1: ACAMS Study Guide, Chapter 2: Risk Assessments, 1

The rules imposed by the Office of Foreign Assets Control (OFAC) for legal entities and persons related to the US are:
Nationals of the US must comply with OFAC rules, regardless of where they are located in the world. This means that US citizens, permanent residents, and entities organized under US law are subject to OFAC sanctions and prohibitions, even if they operate or reside outside the US12.
Any foreign corporation is also penalized if it conducts transactions with sanctioned countries under OFAC rules. This means that non-US entities that engage in trade or financial dealings with OFAC-designated countries, entities, or individuals are liable to face civil or criminal penalties, as well as secondary sanctions that could restrict their access to the US market or financial system34.
The other options are not correct, because:
A subsidiary of a legal entity of the US, which is formally registered in a foreign country, is not exempt from OFAC rules. This means that foreign-incorporated entities that are owned or controlled by US persons or entities are also subject to OFAC sanctions and prohibitions, unless they are specifically authorized or licensed by OFAC12.
A foreign individual visiting the US for a short vacation is not obligated to follow OFAC rules. This means that non-US persons who are temporarily present in the US are not subject to OFAC sanctions and prohibitions, unless they are involved in transactions that have a US nexus or violate other US laws.
The head office of a foreign legal entity which has a branch in the US does not need to comply with OFAC rules. This means that non-US entities that have a presence or operation in the US are not subject to OFAC sanctions and prohibitions, unless they are involved in transactions that have a US nexus or violate other US laws.
References:
ACAMS CAMS Certification Video Training Course - Exam-Labs3
Exam CAMS: Certified Anti-Money Laundering Specialist (the 6th edition)4 ACAMS Study Guide for the Certification Examination, 6th Edition, Chapter 7, page 147:
https://www.acams.org/wp-content/uploads/2019/08/ACAMS-Study-Guide-6th-Edition-Chapter-7.pdf ACAMS Study Guide for the Certification Examination, 6th Edition, Chapter 7, page 148:
https://www.acams.org/wp-content/uploads/2019/08/ACAMS-Study-Guide-6th-Edition-Chapter-7.pdf ACAMS Study Guide for the Certification Examination, 6th Edition, Chapter 7, page 149:
https://www.acams.org/wp-content/uploads/2019/08/ACAMS-Study-Guide-6th-Edition-Chapter-7.pdf

The audit function should report to the audit committee of the board of directors (or similar oversight body) and independently evaluate the risk management and controls of the bank through periodic assessments, including the adequacy of the bank's controls to mitigate the identified risks, the effectiveness of the bank's staff's execution of the controls, the effectiveness of the compliance oversight and quality controls and the effectiveness of the training.