Vehicles equipped with air brakes must have which of the following?

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

Vehicles equipped with air brakes must have an air pressure gauge to show the pressure available for braking. This gauge is crucial as it informs the driver of the air system's current pressure level, indicating whether there is enough pressure in the air tanks to activate the brakes effectively. Adequate air pressure is essential for safe operation; too low a pressure level can result in brakes that fail to engage or operate properly, posing serious safety risks while driving. Regular monitoring of the air pressure gauge allows drivers to take appropriate action if they notice the pressure dropping, which could indicate a potential leak or other malfunction in the air brake system. This proactive approach to vehicle safety is key to preventing accidents and ensuring the reliability of braking performance when needed.