Which of the following must banks report as a suspicious activity?

Banks are required to report suspicious activities that may involve money laundering or other illicit financial activities. Large cash transactions can trigger suspicion for several reasons, including the potential for structuring (smurfing) where individuals break up large amounts of cash into smaller deposits to evade reporting thresholds. Because large cash transactions can be indicative of criminal behavior, such as drug trafficking or terrorism financing, they must be scrutinized and reported if they raise suspicion.

In contrast, high-frequency trading is generally a legitimate trading activity characterized by executing a large number of orders at very fast speeds but does not inherently indicate suspicious behavior. Regular deposit patterns do not typically raise red flags as long as they are consistent with a person's known financial activities. Long-term savings accounts usually represent stable savings habits and lack the characteristics that would necessitate reporting as suspicious. Therefore, large cash transactions are the correct choice as they are closely monitored and can often lead to regulatory scrutiny if they appear unusual or have no clear source.