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The correct answer is the emergency brake system, which is designed to engage automatically when air pressure is lost. This is a safety feature of air brake systems in commercial vehicles. When air pressure drops below a certain threshold, the spring brakes are activated, applying the brakes to bring the vehicle to a stop. This helps prevent loss of control and potential accidents due to brake failure.

The emergency brake system is specifically engineered to operate independently of the main braking system, which relies on air pressure to function properly. This independence is critical in emergencies when the primary braking system may fail. By utilizing spring pressure to activate the brakes, the emergency system ensures that the vehicle can still be stopped safely, promoting overall safety.

Other types of brake systems mentioned have different functions. The service brake system is the primary braking system used during normal driving and relies on air pressure to function. The parking brake system is utilized to keep the vehicle stationary when parked, and while it may also use spring pressure, it does not engage automatically in the event of air pressure loss as the emergency system does. The anti-lock brake system is designed to prevent wheel lock-up during hard braking but does not engage brakes due to air pressure loss.