What component must be activated to release the spring brakes?

Answer: B — The air supply line must be pressurized

To release the spring brakes, the air supply line must be pressurized. This is fundamental to the operation of spring brakes in air brake systems. When the air pressure in the system is sufficiently elevated, it sends air to the brake chambers, which pushes against the springs that hold the brakes in the engaged position, thus releasing them. In contrast, the emergency brake lever, manual release valve, and parking brake switch are mechanisms that interact with the braking system but are not the primary components responsible for releasing the spring brakes. The emergency brake lever typically engages the emergency brakes rather than releasing them. The manual release valve can be used in certain situations to release the brakes but is not the standard operating method. On the other hand, the parking brake switch may control the overall operation of the parking brake system but also doesn’t directly relate to the release function without pressurized air. Therefore, pressing the air supply line is the key action required for the effective release of the spring brakes, ensuring proper vehicle operation and safety.