BOTH INTERNATIONAL & INLAND You are underway in fog and you hear one prolonged blast followed by two short blasts. This signal indicates which vessel?

Correct Answer: Option D — A towing vessel

Rule 35(c) specifies that a vessel engaged in towing or pushing another vessel shall sound one prolonged blast followed by two short blasts at intervals of not more than two minutes. This signal identifies the vessel's status as a towing unit to other vessels navigating in restricted visibility.