In the event of an air brake failure, what emergency feature allows you to stop the vehicle?

Answer: B — The spring brake mechanism.

The spring brake mechanism is a critical feature in air brake systems designed to provide a fail-safe method of stopping a vehicle in the event of an air brake failure. This mechanism utilizes powerful springs to apply the brakes automatically when there is a loss of air pressure. When the air pressure drops below a certain threshold, the spring brakes engage, ensuring that the vehicle slows down or comes to a stop without relying on the air brake system, which may have failed. This is particularly important for large vehicles like trucks and buses, where maintaining control and safety is paramount. In contrast, while the emergency brake pedal and the parking brake lever offer additional braking options, they do not serve the same purpose during a complete air brake failure as the spring brake mechanism. The anti-lock brake system is designed to prevent wheel lock-up during hard braking but does not function to stop the vehicle in the absence of air pressure. Thus, the spring brake mechanism is the correct answer for an emergency stopping feature in the case of air brake failure.