Final answer:
A rapid ringing of a bell heard in fog signals the presence of a vessel at anchor, as prescribed by both International and Inland Navigation Rules to prevent collisions.
Step-by-step explanation:
While underway in fog, hearing a rapid ringing of a bell ahead indicates A) A vessel at anchor. According to the International and Inland Navigation Rules, a vessel at anchor must ring the bell rapidly for about 5 seconds at intervals of not more than 1 minute to signal its position in conditions of reduced visibility. This is a safety measure to alert other vessels of its presence to prevent collisions.
The rapid ringing of a bell ahead while underway in fog can indicate a vessel in distress. This is because vessels in distress often use a bell or horn signal to alert nearby vessels of their situation and to request assistance. By hearing this signal, you should be prepared to respond and offer aid to the distressed vessel.