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.

The key concept is signaling diving operations during daylight when you can’t display the usual day shapes. In that situation, the vessel should display a rigid replica of the International Code flag Alpha. This is a standardized, easily recognized signal that divers are in the water and work is being conducted, so other vessels know to keep well clear and proceed with caution. The other flags mentioned don’t convey this specific diving-operations status, so they aren’t appropriate for signaling in this scenario.

In nautical signaling, colors of lights indicate different vessel activities. A towing operation uses a yellow (amber) light to show that the vessel is towing or is being towed. This distinct color signals other boats to exercise caution and expect unusual maneuvering or lines. The standard navigation lights—red for port, green for starboard, and white for the stern—are different from the towing light, which is specifically yellow to denote towing. Because of the potential for restricted maneuverability and the risk of lines, the yellow towing light is used to clearly communicate this status to nearby vessels.

In the Rules of the Road, vessels are classified by how they are propelled. A sailing vessel is propelled by sails alone and not by any engine, while a power-driven vessel is propelled by machinery. If a vessel uses both sails and an engine to move, the engine provides propulsion, so it falls into the power-driven category. That classification is important because the applicable rules—such as who must keep clear or how predictable actions are—are written with these categories in mind. A sailing vessel would follow the sailing-specific rules, but once engine propulsion is present, the vessel is governed by the power-driven rules. The other descriptions don’t fit because they describe vessels that are either sailing-only, not propelled at all, or use a term not used for the formal regulatory classification in the Rules.