How can a driver confirm that the brakes are applied when parking?

Answer: D — The spring brakes automatically engage when the air pressure drops

When parking a vehicle equipped with air brakes, drivers can rely on the fact that the spring brakes automatically engage when the air pressure drops. This feature is crucial for ensuring that vehicles remain securely parked, as the spring brakes act as a fail-safe. When the air pressure in the brake system falls below a specified level, the spring brakes are activated to prevent the vehicle from rolling away. This automatic engagement alleviates the need for drivers to manually check the brake position each time they park, as the system is designed to activate without requiring manual intervention. This characteristic is essential for maintaining safety and stability, particularly in heavy vehicles where the risk of rolling can have serious consequences. In contrast, while illuminating brake lights indicates that the brakes are engaged to others, it does not confirm the physical status of the brakes themselves. Moreover, manually engaging the brakes may not be effective in ensuring the vehicle does not roll unless the spring brakes have engaged. Similarly, relying on the vehicle rolling as a measure of brake application is unsafe and not a sound practice for securing a parked vehicle.