Which navigation signal indicates a vessel that is approaching the channel and may restrict its ability to maneuver?

Answer: D — Single all-round red light

The navigation signal indicating a vessel that is approaching the channel and may restrict its ability to maneuver is denoted by a single all-round red light. This signal is specifically used to inform other vessels that the vessel displaying it is constrained by its draft or is in a position where it cannot safely navigate or maneuver. The use of a single all-round red light serves as a clear and easily recognizable signal to nearby vessels, alerting them of the situation and the need for caution in navigating near the restricted vessel. In contrast, other signals mentioned correspond to different situations or vessels. A prolonged blast followed by short blasts, for example, typically relates to communication between vessels rather than indicating a navigation state. The flashing yellow light is often used for other specific situations but does not indicate a restriction in maneuverability. Two short blasts followed by one prolonged blast is commonly used to signal an intention to maneuver or signal, which is not representative of restricted ability. Thus, the single all-round red light effectively communicates the limited maneuverability of the vessel while approaching the channel.