What must air brake equipped vehicles have to indicate air pressure?

Answer: A — supply pressure gauge.

Air brake equipped vehicles are required to have a supply pressure gauge that indicates the air pressure within the system. This gauge is crucial for drivers to monitor the air pressure level, ensuring that it stays within safe operating limits. Insufficient air pressure can lead to brake failure, making the supply pressure gauge an essential safety feature. Having such a gauge allows the driver to be aware of the air system's status and respond appropriately, such as performing necessary maintenance or addressing any issues before they become critical. The presence of this gauge helps in maintaining vehicle safety standards and is a fundamental requirement in the design of air brake systems.