What signal must be given when backing up a vehicle?

Using a spotter or going in reverse with caution while checking mirrors is essential for ensuring safety when a vehicle is backing up. This action allows the driver to maintain awareness of the immediate surroundings, which is vital in preventing collisions with pedestrians, other vehicles, or obstacles that may not be visible in mirrors alone. Employing a spotter can enhance visibility from outside the vehicle, providing additional guidance and a clear view of any potential hazards behind the vehicle.

The other choices do not adequately address the comprehensive safety measures required during the backing-up process. Sounding the horn continuously may alert others but does not ensure the driver's awareness of their immediate surroundings. Raising a vehicle's flag is not a standard practice for signaling during reverse maneuvers and might be unnecessary in most situations. While turning on all vehicle lights can increase visibility, it does not substitute the need for a spotter or careful mirror checks to ensure a safe backing procedure.