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

When anchoring a 25-meter vessel at night, the requirement is to display one all-round white light. This all-round white light serves as a visibility marker for other vessels, helping to indicate that the boat is at anchor and not underway. The purpose of this light is to ensure safe navigation around the anchored vessel for other boaters, as it provides a clear indication of its status.

Displaying only one all-round white light is sufficient for a vessel of this size as it complies with the regulations under the International Regulations for Preventing Collisions at Sea (COLREGs). This regulation is designed to prevent accidents by ensuring that vessels can easily identify each other, especially in the dark.

The options involving additional lights, such as sidelights or a stern light, are not necessary when at anchor and may create confusion regarding the vessel's status. Therefore, having just the one all-round white light is the correct and sufficient requirement for a 25-meter vessel at night.