What must a vessel do when it hears a signal for a vessel that is overtaking?

Answer: C — Alter course to avoid collision

Hearing a signal from a vessel that is overtaking tells you another vessel is in the process of passing you. The safest, proper action is to take positive steps to increase separation and ensure the overtaking vessel can pass well clear. Altering course to avoid collision gives you that room and reduces the risk of a close-quarters incident. Keeping your course would leave less space for the passing vessel and could lead to danger, while ignoring the signal or answering with a blast does not provide the necessary maneuvering guidance.