When is the horn not required to be operational?

The correct choice clarifies that the horn must always function properly on a vehicle. The horn is an essential safety feature designed to alert other drivers, pedestrians, and animals of a vehicle's presence, especially in situations where visibility may be impaired or a warning is necessary to prevent an accident.

Having a functional horn contributes significantly to road safety by allowing the driver to communicate in emergency situations. The presence of other warning lights does not substitute or replace the horn's warning capabilities. Therefore, regardless of whether the car is in motion, securely mounted, or whether other warning lights are operational, the horn must always be in working order to fulfill its critical role in vehicle safety.