When can a vessel display a single all-round light at the top of the mast?

A vessel can display a single all-round light at the top of the mast when it is displaying the prescribed light signals for its type, which corresponds to the correct option. This configuration typically applies to vessels that are at anchor, vessels that are engaged in certain operations, or those under specific circumstances that necessitate signaling to indicate their operational status.

In instances where a vessel is required to show this light, it serves to inform other vessels of its presence and operational status. The all-round light is a white light that provides visibility in all directions, which is especially crucial in situations where the vessel is either stationary or operating in a manner that would not require more complex lighting configurations.

The other options highlight scenarios where the lighting configurations differ. For example, when anchored, a vessel would usually show specific anchor lights, typically all-round white lights at different heights or additional signal lights if required. Trawling and sailing under power also have distinct lighting requirements that indicate the operational mode of the vessel, ensuring other navigators understand its activities and status. Therefore, the all-round light is specifically used under prescribed circumstances like signaling.