On what principle does a disc brake operate?

A disc brake operates on the principle of hydraulic pressure. When the driver presses the brake pedal, hydraulic fluid is forced through the brake lines into the brake caliper. This action causes the brake pads to clamp down on the rotor (the disc), creating friction that slows down or stops the vehicle.

In the context of option C, air pressure in a compressed air system acts on a brake chamber, which then applies force to a slack adjuster. This movement activates the brakes by pushing the brake pads against the rotor. This mechanism combines the principles of air pressure and hydraulic pressure to effectively engage the braking system.

While other options may mention relevant concepts, they do not accurately describe how disc brakes function. Hydraulic pressure is the crucial factor in creating the necessary force to slow or stop the vehicle, making it an essential aspect of disc brake operation.