In terms of operation, what is required of the horn?

The requirement for the horn in a commercial vehicle is that it must function properly. This means the horn needs to be in good working condition and able to emit a loud, clear sound when activated. This functionality is essential for safety reasons, as the horn is a crucial tool for alerting other drivers, pedestrians, and animals of the vehicle's presence, especially in situations where visibility might be hindered.

A properly functioning horn allows the driver to communicate effectively with others on the road, thus helping to prevent accidents and promote safe driving practices. The emphasis on proper functioning ensures that drivers can rely on this safety feature under all necessary circumstances without the risk of it failing to work when it is needed most.

In contrast, other options imply limitations or inadequacies in horn performance, which would not meet safety standards set forth for commercial vehicles. Thus, only a horn that is fully operational is acceptable in terms of operational requirements for safe driving.