What sound signal indicates a vessel is aground?

The sound signal indicating that a vessel is aground consists of three rapid strokes of the bell followed by three long blasts. This specific sequence of sound signals is recognized internationally and is used to alert nearby vessels of the situation. A vessel that is aground may pose a danger to navigation, and this sound signal serves as a warning for other vessels to keep their distance while assisting or avoiding the aground vessel.

Other options describe different sound signals that correspond to other specific scenarios. For instance, one blast every minute is used to indicate a vessel that is under way but not making way, while two short blasts indicate a vessel's intention to pass on the starboard side. Four long blasts do not represent a vessel aground; instead, they are utilized to signify that a vessel is not under command. Each sound signal has specific meanings in marine navigation, which helps ensure safety on the water.