When is it permissible to use your vehicle's horn?

Using your vehicle's horn as a reasonable traffic warning is the correct answer because the primary purpose of a vehicle's horn is to enhance safety on the road. Horns are designed to alert other road users to potential dangers; for instance, warning a pedestrian who may not have noticed your approach or signaling to another driver who might not see you. This use is intended to prevent accidents and promote safe driving practices.

The other options, such as using the horn to express frustration, greet others, or alert a friend, do not align with the intended safety function of a vehicle's horn. These actions can lead to confusion among other road users and may even contribute to unsafe situations, making it essential to reserve the horn's use for legitimate traffic warnings.