What action is required if financial institutions identify suspicious transactions?

Financial institutions are required by law to report any suspicious transactions to the appropriate authorities. The Anti-Spam Authority (A) deals with unsolicited and misleading electronic communication, which is unrelated to financial transactions. Reporting to the police (B) may result in further investigation, but it is not the specific action required for suspicious financial transactions. Submitting an Anti-Money Laundering Report (D) is related to money laundering activities, and while suspicious transactions may be a part of such activities, it is not the specific action required in this scenario. Therefore, the correct action to take if financial institutions identify suspicious transactions is to submit a Suspicious Transaction Report (C).