What should a vessel display when at anchor according to navigation rules?

When a vessel is at anchor, the navigation rules require it to display one all-round white light. This light must be visible from all directions and is essential for signaling the presence of the anchored vessel to other mariners, especially during periods of reduced visibility such as nighttime or in poor weather conditions. The purpose of this requirement is to enhance safety by preventing collisions and ensuring that other vessels can recognize the anchored vessel.

Displaying one all-round white light signifies to others that the vessel is not under command and is securely anchored. This is a standard safety measure that promotes awareness and caution among nearby vessels. While the other options might seem plausible, they do not conform to the specific regulation that mandates the use of one all-round white light when a vessel is at anchor. Thus, option B aligns correctly with the navigation rules for vessels at anchor.