The correct answer is B) sailboat underway. At night, a sailboat underway is required to display a single green sidelight, which indicates the vessel's starboard (right) side. This light configuration is defined in the Navigation Rules, both the International and Inland Navigation Rules, which govern the lighting requirements for vessels at sea. The other answer choices are incorrect because a vessel at anchor (A) would display an all-around white anchor light, a small motorboat underway (C) would display red and green sidelights, and a vessel drifting (D) would still be required to display the appropriate navigation lights, such as sidelights, to indicate its presence and orientation.