What action applies the parking brake in an air brake system?

Answer: B — Using the parking brake control

Using the parking brake control is the correct action to apply the parking brake in an air brake system. This control specifically enables the driver to engage the parking brakes, which are designed to hold the vehicle stationary when it is parked. In air brake systems, the parking brake is typically applied by utilizing a lever or button that releases air from the brake chambers, allowing springs to push the brake shoes against the drums or discs. Other actions, such as releasing pressure from the air tanks or pressing the brake pedal, do not apply the parking brake. Releasing pressure from the air tanks would instead deactivate the air brake system, while pressing the brake pedal activates the service brakes used for stopping the vehicle while in motion. Automatic engagement typically involves safety mechanisms to prevent the vehicle from rolling when it comes to a stop but does not specifically relate to the manual operation of the parking brake control. Understanding the proper use of the parking brake control is crucial for safely securing a vehicle when parked.