Which lighting is used as an anchor light when sidelights are extinguished?

An all-round white light is the designated lighting for an anchor light when sidelights are extinguished. This type of light is used to indicate the presence of an anchored vessel to other boats and is required by navigation rules, particularly during nighttime or in low visibility conditions. The all-round white light is visible from all directions, ensuring that the vessel can be seen by approaching boats, which enhances safety on the water.

The other options do not serve the same purpose in this context. The sternlight, for example, is a white light located at the back of the vessel and is only visible from behind, making it insufficient for indicating the vessel's position when at anchor. The masthead light serves a different function as it is typically used for vessels underway and indicates the forward direction, while the red port light is one of the sidelights used to indicate the vessel's port side during nighttime navigation and isn't visible from all angles necessary for an anchor situation.