What is the requirement for lights on a vessel at anchor as specified by COLREGs?

Answer: A — Only an all-round white light

When a vessel is at anchor, the COLREGs stipulate that it must exhibit an all-round white light visible in all directions. This requirement is crucial for ensuring that other vessels can easily identify the position of the anchored vessel, especially in low visibility conditions or at night. The all-round white light serves as a straightforward indicator that the vessel is not under way, helping to prevent collisions. The other options involve colored or flashing lights, which are not suitable for vessels at anchor. The use of colored lights or flashing signals may imply different navigational statuses, such as maneuvering or distress, which could create confusion regarding the vessel's situation and lead to potential hazards on the water. Thus, displaying an all-round white light is the correct and required method to indicate a vessel anchored in accordance with international regulations.