A sailboat is operating under sail at night. Which of these lights should be used?

A sailboat operating under sail at night must display specific lights to indicate its presence and actions to other vessels. The correct choice is to use a white sternlight, which is positioned at the rear of the boat. This sternlight is essential for providing visibility to vessels approaching from behind.

In addition to the white sternlight, a sailboat should also have red and green sidelights, which indicate the port (left) and starboard (right) sides respectively when underway. However, since the question specifically asks for the lights used while sailing at night, the emphasis is on the sternlight's role in night visibility and navigation.

The other options, such as red or green sidelights and a flashing light, are not suitable by themselves. Sidelights serve specific functions, and a flashing light is typically used in emergency situations, not for standard navigation. Hence, the white sternlight is the required light for a sailboat operating solely under sail at night.