What is the additional sound signal used by an anchored vessel for warning?

The correct response is two prolonged blasts followed by two short blasts. This specific sound signal is used by a vessel that is anchored to indicate its presence to other vessels in the area, effectively serving as a warning signal.

The two prolonged blasts followed by two short blasts help differentiate an anchored vessel from other boating activities. This signal is particularly important because it conveys to nearby vessels that the anchored vessel is stationary and not under way, allowing them to navigate safely around it and avoid potential collisions.

Using sound signals in this manner is crucial for safety and navigation, especially in poor visibility conditions. This practice aligns with the regulations set forth in the COLREGs, which emphasize the importance of proper signaling to prevent accidents at sea.