Which lights must a sailing vessel underway display?

A sailing vessel underway must display sidelights and a sternlight to ensure proper visibility and identification to other vessels. Sidelights, which are red and green, indicate the port (left) and starboard (right) sides of the vessel, enabling other boats to determine the direction the sailing vessel is heading. The sternlight is white and is positioned at the rear of the vessel, which helps indicate the presence and direction of the vessel from behind.

This combination of sidelights and a sternlight is crucial for nighttime navigation and is a standard requirement under the International Regulations for Preventing Collisions at Sea (COLREGs). It enhances safety by making the sailing vessel visible to others and allowing for better situational awareness during dark or low-visibility conditions. The other options do not meet the full requirement for a sailing vessel underway, as having only a sternlight or sidelights alone would not provide sufficient indication of the vessel's position or direction to other mariners.