What safety feature must be present on all vehicles equipped with air brakes?

Answer: B — low air pressure warning signal

The presence of a low air pressure warning signal is essential for all vehicles equipped with air brakes because it serves as a critical safety feature to alert the driver when the air pressure in the braking system falls below a safe level. Air brakes rely on a specific air pressure range to function effectively; if the pressure drops too low, the brakes may fail to engage or respond as needed, endangering the safety of the vehicle and its occupants. This warning signal typically activates when the air pressure falls below a predetermined threshold, prompting the driver to take immediate action, like checking the system or reducing speed, thereby preventing potential accidents due to brake failure. By ensuring that these warning systems are operational, vehicles can maintain a high standard of safety on the road. The other options pertain to different aspects of vehicle safety but do not fulfill the same critical role that a low air pressure warning signal does in monitoring brake performance.