Explanation/Reference:

this activity is unusual and inconsistent with the expected behavior of an accounting firm's operational account. The account should normally receive fees from clients that are proportional to the services rendered and reflect the market rates. Receiving international payments that are substantially higher than other fees could indicate that the account is being used to launder money or finance terrorism by disguising illicit funds as legitimate income. This could trigger a red flag for the bank and prompt a comprehensive review of the account.
References:
ACAMS Study Guide 6th Edition, Chapter 1, Section 1.6, page 24: "Methods to launder money used in banks and other deposit taking institutions".
ExamTopics, Question 446: "An accounting firm opened an account at a bank that is intended to be used as the operational account for the business. After a few months, a comprehensive review of the account was triggered due to unusual activity on the account. Which most likely triggered the review?"

Under USA PATRIOT Act Section 311, the U.S. government can impose special measures against a foreign jurisdiction, institution, or transaction that is of primary money laundering concern1. These measures can range from requiring additional recordkeeping and reporting to prohibiting or restricting the opening or maintaining of correspondent or payable-through accounts for the designated entity2. A payable-through account is a type of correspondent account that allows customers of a foreign bank to access the U.S. financial system by writing checks or making wire transfers from the foreign bank's account at a U.S. bank3.
In this case, the U.S. government has enacted special measures against a designated entity that has a payable- through account with an EU bank, which in turn has a correspondent banking relationship with a U.S. bank.
One of the possible actions that the U.S. bank might be required to take regarding the EU bank's services for the designated entity is to obtain additional information about customers permitted to use this account. This is to ensure that the U.S. bank can identify and monitor the transactions and activities of the designated entity and its customers, and to prevent any money laundering or terrorist financing risks4.
The other actions listed are not likely to be required by the U.S. government under Section 311. Ensuring the designated entity's confidential information is not shared with other entities is not a special measure, but a general obligation of any financial institution under privacy laws. Verifying that the EU bank serves the designated entity is not a relevant action, since the U.S. government has already designated the entity as a primary money laundering concern. Performing enhanced due diligence on the EU bank is not a specific action related to the payable-through account, but a broader requirement for any correspondent account under Section 312 of the USA PATRIOT Act5.
References:
1: USA PATRIOT Act | FinCEN.gov1 2: 311 Actions | U.S. Department of the Treasury2 3: Overview of Correspondent Banking and "De Risking" Issues - CRS Reports3 4: U.S. TREASURY DEPARTMENT OFFICE OF PUBLIC AFFAIRS4 5: Fact Sheet: Overview of Section 311 of the USA PATRIOT Act5

Non-compliance with AML regulatory obligations can have severe consequences for financial institutions.
Among these, the most severe consequence is the loss of license. When a financial institution fails to meet AML requirements, regulators may revoke its license to operate. Losing the license effectively shuts down the institution's ability to conduct business, impacting its existence and operations significantly12.
References:
1. Unit21 AML: 8 AML Penalties, Fines, and Sanctions + Examples You Should Avoid
2. Financial Crime Academy: Ensuring Compliance: The Power Of AML Regulatory Reporting