You are underway, in fog, when you hear a whistle signal of one prolonged blast followed by two short blasts. This signal could indicate a vessel _____________.

The correct answer is that the signal indicates a vessel not under command. In navigation, vessels that are unable to maneuver or are restricted in their ability to navigate must signal their situation to other nearby vessels to avoid collisions. The prescribed sound signal for a vessel not under command in fog or restricted visibility is one prolonged blast followed by two short blasts.

Understanding the context of the other options helps clarify why the specific option about a vessel not under command is correct. While a vessel being towed or aground have their own sound signals that they should use—such as two prolonged blasts for vessels being towed, and a different signal for vessels aground—those are not the sounds indicated by the signal in question. Therefore, the specific combination of one prolonged blast followed by two short blasts is distinctly associated with vessels that are not able to navigate due to being incapacitated in some manner. This distinction is vital for mariners to effectively communicate their status and ensure safe passage in foggy conditions.