What type of sound signal should a vessel at anchor use to signal its presence to nearby vessels during fog?

Answer: C — one prolonged blast every minute

When a vessel is at anchor and there is reduced visibility, such as during fog, it is required to sound a signal to alert nearby vessels of its presence. The correct signal in this situation is one prolonged blast every minute. This specific sound signal is recognized internationally as a means for anchored vessels to notify others of their position, thereby helping to prevent collisions or other dangerous situations in areas where visibility is compromised. The prolonged blast is distinct from shorter blasts and communicates effectively that the vessel is stationary at anchor. In contrast, other options either indicate different navigation scenarios or do not conform to the requirements for a vessel at anchor. Understanding the correct use of sound signals is essential for safe navigation and compliance with maritime rules.