What signal must be sounded by a vessel towing a mobile offshore drilling unit through an area of restricted visibility?

Correct Answer: Option C — One prolonged and two short blasts

COLREG Rule 35(c) mandates that a power-driven vessel when towing or pushing, if restricted in her ability to maneuver, shall sound one prolonged and two short blasts at intervals of not more than 2 minutes in restricted visibility, ensuring adequate acoustic warning to surrounding traffic.