Before backing under a trailer after connecting the airlines, what should you do?

Supplying air to the trailer system before backing under a trailer is important because it ensures that the trailer's braking system is properly charged and functioning. When you supply air to the trailer, it activates the braking system, allowing you to test its functionality. This is crucial because a properly functioning brake system ensures that the trailer will be secured once connected.

By following this procedure, you are taking necessary precautions to prevent accidents that could occur if the brakes are not engaged upon backing under the trailer. It is also good practice to ensure that the brakes are operational before any movement occurs, contributing to the overall safety of the operation.

In this context, other options may not ensure effective brake operation or could lead to unsafe situations when connecting to trailers. Taking these steps before backing under provides a higher level of safety and preparedness, which is vital when working with combination vehicles.