The correct answer is A) cannot be made ready immediately and vessels shall stand clear. A flashing red light displayed at a single lock indicates that the lock is not ready for immediate use. Vessels must stand clear and not approach the lock until the light changes, signaling that the lock is ready. This is the standard meaning of a flashing red light at a lock, as per the U.S. Coast Guard regulations. The other options are incorrect because B) suggests vessels may approach even though the lock is not ready, C) indicates the lock is ready when the light is flashing red, and D) states vessels must stand clear when the lock is actually ready for use, which is the opposite of the correct meaning.