Which of the following signifies an unplanned approach back to the runway?

The correct answer, indicating an unplanned approach back to the runway, is a flashing red light. In aviation signaling, a flashing red light is commonly used to communicate that an aircraft should not land. When pilots see this signal, it often indicates that there may be an emergency, or the aircraft is unable to safely land at the moment, thus necessitating an unplanned or aborted approach.

This signaling is crucial in air traffic control and safety protocols, as it helps manage the flow of air traffic and ensures that pilots receive timely and clear instructions regarding their landing status. Understanding this will aid in maintaining safety in operational environments where timely decision-making is vital.