A vessel at anchor must display __________.

A vessel at anchor is required to display one all-round white light. This light serves as a signal to other vessels that the anchored vessel is not under way and helps prevent collisions in situations where visibility may be reduced. According to the International Regulations for Preventing Collisions at Sea (COLREGs), the single all-round white light should be exhibited at a height where it’s visible from all directions around the vessel.

The two all-round white lights mentioned in one of the options are not needed for a vessel at anchor, as the regulations specifically state the requirement is for one light only. While a vessel may have additional lights for other operational conditions (like navigation), when at anchor, the singular all-round white light is sufficient to indicate its status. Similarly, the options suggesting no lights or only a stern light do not fulfill the legal requirements for indicating an anchored vessel's presence.