What is the indication for signaling a vessel that is being towed?

The correct answer relates to the specific sound-signaling requirements set forth in the International Regulations for Preventing Collisions at Sea (COLREGs). When a vessel is being towed, the appropriate signals consist of one prolonged blast followed by three short blasts. This sound signal effectively communicates to nearby vessels that one is being towed, helping to prevent collisions due to a lack of awareness about the towing situation.

The use of one prolonged blast signals the presence of the vessel being towed, while the three short blasts indicate the specific nature of the situation, making it clear that the vessel is not maneuvering under its own power. This combination is necessary to ensure that other vessels understand the type of activity occurring and to facilitate safe navigation in close waters.

Other options either do not conform to the established signaling standards or represent different sailing conditions that do not apply to towing vessels. Understanding these signals is crucial for ensuring communication and maritime safety while on the water.