What is the minimum PSI indicated by a gauge or device at the rear of the train?

Answer: C — 75 PSI

The minimum PSI indicated by a gauge or device at the rear of the train is 75 PSI. This value is significant because it ensures that there is adequate air pressure in the brake system to effectively apply the brakes on the train. A reading of 75 PSI is typically the standard threshold where all the brakes can be assured to be functional and responsive under normal operating conditions. Maintaining this level of pressure helps to ensure safety in train operations, as it provides the necessary force to activate the braking mechanisms properly and enhances the control of the train during its movement. If the pressure were to drop below this level, it could jeopardize the effective functioning of the air brake system, potentially leading to safety risks during transit.