The correct answer is D) not under command. The illustration D084RR shows a vessel exhibiting a red light over a white light, which indicates that the vessel is "not under command" as per the International Regulations for Preventing Collisions at Sea (COLREGS). A vessel not under command is one that, through some exceptional circumstance, is unable to maneuver as required by the rules and is therefore unable to keep out of the way of another vessel. The other answer choices are incorrect because a towing vessel would exhibit different lights, a dredging vessel would exhibit different lights, and a vessel showing improper lights would not be exhibiting the specific lights shown in the illustration.