What must air brake equipped vehicles have?

Answer: C — supply pressure gauge and warning signal

Air brake equipped vehicles are required to have a supply pressure gauge and warning signal to ensure the driver is aware of the air pressure in the braking system. This gauge provides real-time feedback on the air system's pressure, which is critical for the safe operation of air brakes. If the air pressure drops below a safe level, the warning signal (usually an indicator light or alarm) alerts the driver to a potential issue with the braking system. This feature is essential because insufficient air pressure can compromise braking performance, leading to dangerous situations. Therefore, having a supply pressure gauge and warning signal is not just a regulatory requirement but a vital safety measure that helps prevent brake failure. The other choices do not reflect necessary requirements for air brake systems or pertain to standard practices in safe vehicle operation.