According to Rule 21, what are the lighting requirements for a vessel anchored at night?

The correct choice highlights that a vessel anchored at night is required to display appropriate anchor lights. According to the USCG Rules of the Road, specifically Rule 21, a vessel at anchor must show a white light that is visible all around the horizon. This lighting is crucial for ensuring that other vessels can see the anchored vessel to avoid collisions. The use of anchor lights indicates the vessel's position and presence, especially in low visibility conditions.

When a vessel is anchored, remaining completely dark, as suggested in one of the other options, would not provide any indication to other vessels of its presence, increasing the risk of accidents. Flashing sailing lights or shining a spotlight continuously would not fulfill the specific requirements for vessels at anchor and could also cause confusion or distract other mariners. Thus, displaying the correct anchor lights is essential for safety and compliance with maritime regulations.