What should you check regarding the vehicle's horn?

The correct answer highlights the importance of ensuring that the vehicle's horn functions correctly. A properly working horn is essential for safe driving as it allows the driver to signal their presence to other road users and alert them in potentially hazardous situations. The functionality of the horn is a crucial safety feature that supports communication on the road, helping to prevent accidents and maintain safety.

In a pre-trip vehicle inspection, checking that the horn works as intended ensures that the driver can use this important tool when necessary. Properly functioning equipment is a core aspect of vehicle safety checks and is specifically outlined in safety regulations for commercial vehicles.

The other options do not adequately address the critical requirement for vehicle operation. A rusted horn indicates neglect and may affect performance, a horn that is "not too loud" could imply a malfunction since it needs to be loud enough to be effective, and replacing the horn annually without addressing its function is not a practical approach to vehicle maintenance. These factors emphasize the necessity of a working horn as an essential safety feature rather than focusing on its physical condition or sound level.