Which statement is TRUE concerning fog signals?

The statement that a vessel aground may sound a whistle signal is accurate, as vessels that find themselves aground have specific sound signal requirements to alert others to their condition. According to the International Regulations for Preventing Collisions at Sea (COLREGs), a vessel that is aground must signal with three separate strokes on the whistle, indicating its situation to other vessels. This is essential for safety, especially in conditions of reduced visibility, such as fog.

In contrast, other choices do not align with the rules. Not all vessels must sound a bell, as this is typically a requirement for vessels over a certain tonnage in specific situations, while not every vessel is mandated to have a bell. The requirement for every vessel to sound a horn also does not hold true, since the rules specify sound signals only for certain situations and types of vessels. Lastly, the belief that only powered vessels are required to signal is incorrect since both sailing vessels and other types are subject to sound signal regulations under various circumstances.