What is a suspicious transaction?

The question asks for the definition of a suspicious transaction.

Let's analyze the given options:

• a) "Transaction with unknown parties that do not match the customer's profile." While transactions with unknown parties can be suspicious, this option is too narrow as suspicious transactions can also involve known parties or other factors.
• b) "Pattern of transactions that are inconsistent with the expected business or cannot be explained." This option highlights key aspects of suspicious activity: a pattern (not just a single event), inconsistency with the expected business (deviation from normal activity), and the inability to provide a legitimate explanation. This is a comprehensive definition often used in financial regulations.
• c) "Transactions that are different from the expected profile of the customer." This is a good indicator, but option 'b' is more specific by including "pattern" and "cannot be explained."
• d) "Financial activities that deviate from normal or expected patterns." This is a broad and generally correct definition. However, option 'b' adds the crucial element of "cannot be explained," which is often the ultimate trigger for reporting a suspicious transaction.

Considering the nuances, option b provides a more detailed and actionable definition, emphasizing the "pattern" and the lack of a legitimate explanation, which are critical in identifying and reporting suspicious transactions in a financial context.

The final answer is $\boxed{\text{b}}$