What should you do after connecting the air lines but before backing under the trailer?

Answer: B — Supply air to the trailer system

After connecting the air lines, it is crucial to supply air to the trailer system before backing under the trailer. This step is necessary because it allows the brake system on the trailer to activate properly. Supplying air ensures that the trailer's air brakes are charged and functioning correctly, which is vital for safe operation. If the air system is not supplied with air, the trailer brakes may remain engaged, leading to potential hazards when attempting to back under the trailer. While testing trailer lights, checking the trailer weight, and performing a visual inspection of the trailer are also important steps in the coupling process, they should be done at different times or after ensuring that the trailer's air system is ready. Supplying air to the trailer system is the immediate action required to prepare for a safe connection and to ensure that the braking system will operate effectively during subsequent maneuvers.