Which of the following vessels is required to show a stern light?

Answer: C — vessel at anchor

A vessel at anchor is required to show a stern light as part of its navigation lights configuration. According to the International Regulations for Preventing Collisions at Sea (COLREGs), a vessel anchored must display an all-round white light, which can be positioned at the stern or in a location that makes it visible from all directions. However, for vessels that are longer than 50 meters, they should also show a second all-round white light at the stern to ensure better visibility. Understanding the context of the other vessels helps clarify why they do not require a stern light in the same way. A sailing vessel under sail is typically required to show sidelights and a stern light when not at anchor. A power-driven vessel operates under different light requirements that typically include sidelights and a stern light as well, but they are not specifically focused on the anchor situation. A vessel under tow may have different light requirements depending on its configuration but does not have a direct requirement specifically for showing a stern light because it is often the towing vessel that is responsible for displaying the proper navigation lights in these situations. In summary, the requirement for a stern light is a critical safety regulation for a vessel at anchor, ensuring that it is visible to other vessels and helping to prevent collisions