What navigation lights must be displayed by a sailboat under sail that is 21 ft. in length?

A sailboat under sail that is 21 feet in length must display a combination of navigation lights, which typically include a white stern light and a red and green sidelights. The red light should be displayed on the port side (left) and the green light on the starboard side (right). If the sailboat is not under power, it is not required to display a masthead light, but the sidelights and stern light must be visible. These lights ensure visibility to other vessels during nighttime or low visibility conditions.