When anchoring a 25-meter vessel at night, what must be shown?

When anchoring a vessel at night, the requirement is to display one all-round white light. This helps to indicate the location of the anchored vessel to other nearby vessels, enhancing safety during nighttime navigation. The all-round white light ensures that the vessel is visible from all angles, allowing other mariners to recognize its presence and avoid a collision. Displaying a single all-round white light is compliant with the rules governing the navigation of vessels at anchor, as outlined in the Navigation Rules or International Regulations for Preventing Collisions at Sea (COLREGs).

In this context, the other options do not meet the specific safety requirements for an anchored vessel. For example, two all-round white lights would typically indicate a vessel that is aground, while a white flashing light or an all-round green light serve different purposes altogether in navigational signaling and do not apply to vessels at anchor. Hence, the requirement for an anchored vessel at night is indeed to show one all-round white light.