What combination of signals indicates a vessel not under command?

• A. Red lights in a vertical line
• B. Day shapes only
• C. Red lights in a vertical line and corresponding day shapes
• D. A white light with a diamond

Answer: (C)

When a vessel cannot maneuver as required by the rules, it signals not under command so others know it may not take action to avoid a collision. The standard signal uses both lights and day shapes so the message is clear in any visibility.

Not under command is shown at night by two all-round red lights in a vertical line. During the day, the same vessel displays day shapes — two black balls — also in a vertical line near the foremast. Together, the red lights and the day shapes convey the condition unambiguously in all conditions.

The other options don’t fit because a single element isn’t enough: red lights alone don’t indicate not under command, day shapes alone do, and a white light with a diamond corresponds to a different signal or condition.