Which statement is TRUE concerning fog signals?

The statement indicating that a vessel aground may sound a whistle signal is grounded in the International Regulations for Preventing Collisions at Sea (COLREGs), which outline the proper signals to be sounded by vessels in various situations. When a vessel is aground, it is required to sound a whistle signal to alert other vessels to its presence. This is essential for safety, as an aground vessel might pose a risk of collision to other ships navigating in the area.

In addition to this, regulations specify distinct signaling for vessels at anchor, which are generally required to sound a bell, and for vessels underway, which have different requirements compared to an aground vessel. Furthermore, a vessel restricted in its ability to maneuver must sound their specified signals as well, as per the established guidelines. Thus, knowing the correct requirements for each vessel situation is critical for ensuring safe navigation in reduced visibility conditions like fog.