Before backing under a trailer after coupling a tractor to a semitrailer, what should you do with the air supply?

The correct course of action involves supplying air to the trailer system before backing under the trailer after coupling a tractor to a semitrailer. Supplying air ensures that the trailer's braking system is charged and operational. This is crucial for safety; if the trailer brakes are not engaged properly or the system is not pressurized, it could lead to accidents, such as the trailer rolling away or difficulties in controlling the vehicle while maneuvering.

When the air supply is connected and the system is pressurized, it also allows for the proper function of other systems, such as the trailer's lighting and any air-operated accessories. It's a fundamental safety step that ensures the full functionality of the truck-trailer combination before any further action is taken, particularly given that backing under the trailer requires special attention to ensure everything is secure and operational.

Other options do not address the immediate need to ensure the air supply is functioning correctly and do not contribute to the safety of the operation in the same way as supplying air to the trailer system.