A single towing light will be carried above a vessel's sternlight __________.

Answer: B — only if she is towing astern

A single towing light must be displayed above a vessel's sternlight only when the vessel is actively engaged in towing operations. This is in accordance with the Navigation Rules, which specify the appropriate lighting for vessels in different situations. The towing light serves to indicate that the vessel is towing another vessel or object; it is a critical visual signal for other mariners to understand the activities of the towing vessel. This requirement ensures that there is clear communication on the water regarding the vessel's intentions and actions. The other scenarios presented do not necessitate the display of the towing light. For example, when a vessel is anchored or in a narrow channel, different lighting configurations or requirements are specified. During the day, the need for navigation lights, including the towing light, generally does not apply except in specific circumstances, further distinguishing why the other options do not apply to the question regarding towing lights.