What must a vessel over 100 meters at anchor in fog sound?

Answer: C — rapid ringing of the bell.

A vessel over 100 meters at anchor in fog is required to sound a rapid ringing of the bell for safety and navigational awareness. This sound signal is crucial because it alerts nearby vessels to the presence of the anchored vessel, which may not be visible due to reduced visibility conditions like fog. The bell must be rung for approximately five seconds at intervals of one minute to effectively communicate the vessel's position to others in the vicinity. The choice of a rapid ringing of the bell is specifically prescribed for larger vessels to ensure that their presence is noted, as their size can make them difficult to detect in fog. This regulation enhances overall maritime safety by reducing the risk of collisions or misunderstandings regarding the presence of such vessels at anchor.