Which of the following vehicles is NOT required to have a horn?

Answer: D — None; all need a horn

Emergency vehicles are generally equipped with sirens or other warning devices rather than a standard horn. While they are often required to have audible signals for the purpose of alerting other drivers and pedestrians, this does not mean they must have a traditional horn as found in passenger cars and motorcycles. Passenger cars and motorcycles are required to have horns that sound an appropriate warning. The horn is a necessary safety feature in these types of vehicles to alert others to their presence or to signal for attention. Though all vehicles are subject to specific regulations, emergency vehicles may have exemptions regarding horn requirements because they utilize different sound signals suited to their emergency functions, which can be distinct from the regular horn.