What safety feature prevents the air system from over-pressurizing?

The safety valve is the correct answer because it is a crucial component in an air brake system that prevents over-pressurization. This device acts as a fail-safe mechanism by automatically releasing air from the system when the pressure exceeds a predetermined limit. This helps to prevent potential damage to the components of the air brake system and ensures that the brakes function safely and effectively.

The other systems mentioned do serve important functions but do not directly relate to preventing over-pressurization. For instance, the anti-lock brake system helps maintain vehicle control during hard braking but does not manage air pressure levels. Similarly, the automatic differential assists in vehicle traction but is unrelated to air pressure management in the braking system. The traction control system enhances stability and grip by adjusting power delivery but does not influence the air system's pressure levels. Understanding the specific roles of these components highlights why the safety valve is a vital part of ensuring air brake system integrity.