Which of the following is a requirement for the horn on a vehicle?

The requirement that the horn on a vehicle must have a switch within the driver's reach is important for safety and operational effectiveness. This accessible placement ensures that the driver can easily activate the horn in situations where it is needed, such as alerting other drivers or pedestrians to avoid a potential collision or hazard. It reflects an essential consideration for quick responsiveness in emergency scenarios.

Other options do present relevant considerations about horn functionality, but they do not capture the necessity for direct accessibility by the driver in a way that contributes to immediate situational awareness and safety. Therefore, having the switch within reach is crucial for ensuring that a driver can utilize the horn effectively when necessary.