What must be operational to safely back up a vehicle?

The correct answer is that a backup alarm must be operational to safely back up a vehicle. A backup alarm serves as an important safety feature, alerting pedestrians and other nearby individuals that a vehicle is in the process of reversing. This is particularly important in work environments or areas with limited visibility where there may be people walking nearby. The sound emitted by the backup alarm helps to prevent accidents by ensuring that those in proximity are aware of the vehicle's movement.

While other options such as the rearview mirror, brake lights, and horn play important roles in vehicle safety, they do not have the specific function of alerting others when a vehicle is backing up. The rearview mirror aids the driver in seeing behind the vehicle, brake lights signal stopping, and the horn can be used for various alerts but does not specifically indicate reversing. Hence, the operational status of the backup alarm is critical for preventing collisions and ensuring safe backing maneuvers.