What sound signal should a vessel at anchor sound in fog?

When a vessel is at anchor in fog, it is required to sound one prolonged blast at intervals of not more than one minute. This sound signal is essential for alerting nearby vessels to its presence, as visibility may be significantly reduced in foggy conditions. The prolonged blast serves as a warning, allowing other vessels to take necessary precautions to avoid collisions.

The need for this sound signal when at anchor enhances maritime safety during low visibility situations. It provides a clear indication of a vessel's location and status to other mariners navigating nearby waters, facilitating safer passage.

The other options do not meet the requirements established by navigation rules for vessels at anchor in fog. For instance, one short blast and reciprocating honks do not convey the necessary information to ensure safety. Additionally, remaining silent or producing no sound is inadequate, as it fails to alert other vessels to the presence of the anchored vessel. Thus, the sound signal of one prolonged blast is the correct and appropriate action for a vessel at anchor in such conditions.