What is a critical action to take before initiating a backing maneuver?

Answer: B — Check all mirrors

Before initiating a backing maneuver, checking all mirrors is essential for ensuring safety and awareness of the surroundings. This action allows the driver to see not only the area directly behind the vehicle but also the sides where pedestrians, other vehicles, or obstacles may be located. It is critical to have a clear understanding of the path before making any movements, as this helps prevent accidents and improves overall safety. Engaging the parking brake is important for securing the vehicle when stationary, but it does not provide any information about potential hazards while backing up. Adjusting the GPS could distract the driver and takes focus away from performing the maneuver safely. Sounding the horn may alert others to your presence, but it does not replace the need for visual checks to confirm the area is clear. Therefore, the action of checking all mirrors stands out as the most vital step to take before backing up.