What does the supply pressure gauge indicate?

Answer: B — The pressure in the air tanks

The supply pressure gauge is designed to indicate the pressure within the air tanks of the braking system. This is crucial for determining whether there is adequate air pressure available to operate the air brakes effectively. A sufficient supply of air pressure is essential because it ensures that the brakes will engage properly when needed. Monitoring this gauge helps drivers maintain awareness of their braking system's readiness, allowing them to address any pressure issues before they affect safety on the road.