What does one prolonged blast followed by two short blasts on the whistle indicate while underway in fog?

One prolonged blast followed by two short blasts is a specific sound signal used by vessels to communicate their status while underway in limited visibility conditions, such as fog. This sequence indicates that the vessel is engaged in towing operations.

The prolonged blast serves as an alert to other vessels of the presence of a towing vessel, while the two short blasts help clarify the nature of the situation, indicating that the towing operation may involve another vessel. This signaling protocol is part of the International Regulations for Preventing Collisions at Sea (COLREGs), which help ensure safety by allowing vessels to understand each other's actions and intentions, especially in low-visibility scenarios.

The other options do not correspond to this specific sound signal and are characterized by different combinations of blasts. For instance, a vessel at anchor typically uses a different signal, and a vessel returning to port or under sail would also have distinct sound signals that do not match the one described in the question.