When should vessels sound one prolonged blast?

The correct answer is that vessels should sound one prolonged blast when approaching a bend in a narrow channel or when visibility is restricted. This signal serves a critical purpose in maritime safety, particularly in conditions where visibility is limited, such as fog or during nighttime operations.

Sounding one prolonged blast is a means of alerting other vessels to your presence and indicates your intention to maneuver within the channel, which is important because it helps prevent collisions. In narrow channels, where visibility can be severely compromised, this sound signal informs other vessels that you are approaching and maintaining your course, providing them with the information necessary to take appropriate action.

In contrast, while sounding a blast in fog or when entering a harbor can be relevant, they typically involve a different type of sound signal. Vessels in fog may use a series of blasts as a precautionary measure, while entering a harbor may have specific signals associated with the harbor's own navigation rules. Thus, option B focuses on the correct and clear use of one prolonged blast in specific navigation situations where visibility and maneuverability are compromised.