In restricted visibility, you hear the fog signal of one prolonged blast followed by two short blasts. What type of vessel would sound this signal?

The correct answer indicates that the signal of one prolonged blast followed by two short blasts is used by a power-driven vessel that is underway and making way through the water in restricted visibility. This sound signal is specified in the International Regulations for Preventing Collisions at Sea (COLREGs) under Rule 35, which deals with the use of sound signals in restricted visibility.

When a power-driven vessel is underway and making way, it must signal its presence to other vessels to prevent collisions in conditions where visibility is limited. The prolonged blast serves to alert other vessels of its presence, while the two short blasts further communicate that it is a powered craft that is actively navigating.

The other vessel types mentioned in the options do not use this specific signal under these circumstances. For instance, a fishing vessel would sound a different signal, and a vessel at anchor would sound yet another distinct signal. Similarly, a sailing vessel under sail has its own prescribed signals that differ from those of a power-driven vessel underway. Each category of vessel has sound signals designed to convey specific meanings and intentions, emphasizing the importance of understanding these regulations for safe maritime navigation.