Which of the following must a vessel display to indicate that it is not under command and not making way?

The display of two balls or similar shapes arranged in a vertical line is the correct way for a vessel to indicate that it is not under command and is not making way. This signal is part of the International Regulations for Preventing Collisions at Sea (COLREGS), which set specific requirements to enhance maritime safety and communication between vessels. Specifically, when a vessel is unable to maneuver as required by the rules (such as due to mechanical failure or other emergencies), it must exhibit these shapes to alert other mariners of its status.

This visual signal serves as a clear and universally recognized warning to other vessels in the vicinity, ensuring they understand that the vessel cannot comply with the usual navigation rules. Recognizing this signal helps prevent collisions and promotes safety on the water.

The other options do not convey this specific information about a vessel’s status. For example, a single ball indicates the presence of a moored vessel, while a cone pointing downwards signals a vessel that is constrained by its draft. A single cone pointing upwards is indicative of a sailing vessel under power. Therefore, the requirement of two balls in a vertical line is critical for communication concerning a vessel that is not maneuverable.