What safety device prevents over-pressurization in an air brake system?

Answer: C — Safety valve

The safety device responsible for preventing over-pressurization in an air brake system is the relief valve. This component is designed to automatically release air when the pressure exceeds a predetermined level, ensuring that the system does not sustain damage from excessive pressure. The relief valve is a critical safety feature that protects not only the air brake system but also the overall integrity of the vehicle's braking system. While other components like pressure gauges provide important information about the current pressure levels in the system, they do not actively prevent over-pressurization. Control units manage the operation of the braking system but do not serve the specific function of pressure relief. Similarly, while there may be some confusion around the term "safety valve," it is typically a broader term that can refer to any valve designed for safety purposes, but in the context of air brake systems, the relief valve is the specific device meant for this role.