Which vessel must show two masthead lights in a vertical line?

Answer: D — A vessel not under command

Two white masthead lights arranged in a vertical line are a signaling requirement for power-driven vessels that are 50 meters or longer and underway. The purpose of having two lights one above the other is to ensure visibility and to convey the vessel’s size and status to other mariners at greater distances. Shorter vessels (under 50 meters) follow the same rule for showing a masthead light, but only a single masthead light is required; the second masthead light is not mandatory until the length reaches 50 meters. Applying this to the options: a sailing vessel under sail would not show masthead lights, a power-driven vessel under 50 meters would show only one masthead light, a fishing vessel with gear deployed uses different light patterns, and a vessel not under command shows red lights in a vertical line rather than masthead lights. Therefore, the situation described by the rule—two masthead lights in a vertical line—would apply to a large power-driven vessel 50 meters or more in length, which isn’t clearly represented in the given choices.