What is a supply pressure gauge used for?

A supply pressure gauge is specifically designed to monitor the pressure within the air tanks of a vehicle equipped with air brakes. This is crucial for the safe operation of commercial vehicles, as proper air pressure is necessary for the brakes to function effectively. Maintaining an adequate supply of air pressure ensures that the braking system is reliable and responsive, ultimately enhancing safety.

The other options are unrelated to the function of a supply pressure gauge. Measuring engine temperature pertains to the engine cooling system, indicating fuel level relates to the vehicle's fuel tank, and showing battery voltage is associated with the vehicle's electrical system. Therefore, the correct answer, which highlights the role of a supply pressure gauge in monitoring the air tanks, is essential for understanding its importance in vehicle operation and safety.