What lights must sailboats display when underway at night?

Answer: B — Red and green sidelights and a sternlight

When sailboats are underway at night, they are required to display red and green sidelights and a sternlight. The red and green sidelights indicate the port (left) and starboard (right) sides of the vessel, allowing other boats to determine the sailboat's direction and position in the water. The sternlight, which is white, is positioned at the back of the vessel and serves to make the sailboat visible from behind. Displaying both sidelights and a sternlight is essential for nighttime navigation and collision avoidance, as it provides other vessels with vital information regarding the sailboat’s orientation. Hence, the combination of these lights fulfills the necessary requirements under the International Regulations for Preventing Collisions at Sea (COLREGs) and enhances safety during nighttime operation.