What is the characteristic of a vessel displaying only a white light?

A vessel displaying only a white light indicates that it is either anchored or moored and is not underway. The white light serves as a signal primarily to other vessels in the area, helping to prevent collisions.

When a vessel is at anchor, it typically shows a white light to indicate its position to other mariners. This is a standard practice to ensure safety, as it lets others know that the vessel is stationary. In this context, the characteristic being displayed is understood worldwide as a signal of being at anchor rather than in motion or approaching another vessel head-on.

The other options pertain to different navigation scenarios: being underway would require a specific combination of lights, and displaying a light at the stern of another vessel could imply other colored lights to indicate its direction and status. In the case of approaching head-on, vessels would display multiple navigation lights to indicate their status and maneuvering options, rather than just a solitary white light.