What is the requirement for the vehicle's horn during the inspection?

The requirement for a vehicle's horn during the inspection is centered on its functionality and audibility. The horn must be operable and should sound loud enough to be heard clearly by others, which is crucial for safety on the road. A functioning horn serves as an important signaling device to alert other drivers and pedestrians, helping to prevent accidents.

Regarding the other options, the stipulation about a specific brand is unnecessary since any horn that meets the operability and volume standards is acceptable. The notion that the horn must be replaced with an air horn is incorrect; while air horns are a type of horn, the regulations do not mandate the use of any particular kind. Lastly, stating that it is optional to have a horn contradicts safety guidelines, as having a functional horn is a critical requirement for vehicle inspections.