The correct answer is D) White. According to the U.S. Coast Guard regulations, a mooring buoy, if lighted, must show a white light. This is because mooring buoys are not navigational aids and do not need to follow the standard color scheme for navigation lights (red, green, yellow). The purpose of lighting a mooring buoy is to make it visible at night, so a simple white light is the appropriate choice. The other answer choices are incorrect because A) any color except red or green is too broad, B) blue is not a standard color for mooring buoys, and C) yellow is reserved for special purpose buoys, not mooring buoys.