What must vehicles with air brakes have?

Answer: B — An air pressure gauge to show the pressure available for braking.

Vehicles equipped with air brakes are required to have an air pressure gauge to indicate the available air pressure for braking. This gauge is crucial for drivers to monitor the air system's functionality and ensure there is enough pressure to activate the brakes effectively. If the pressure falls below a certain level, the brakes may not function properly, posing a significant safety risk. Having a reliable air pressure gauge allows the driver to make timely decisions, such as performing a pre-trip inspection or identifying potential issues with the air brake system while on the road. This emphasis on monitoring air pressure is critical for the safe operation of commercial vehicles fitted with air brakes.