What should be done to the trailer before backing the tractor under it?

Answer: A — The trailer should be fully raised

To properly back a tractor under a trailer, the trailer should be fully raised. This ensures that the fifth wheel on the tractor can adequately engage with the kingpin of the trailer. Raising the trailer allows the tractor to align and lower onto it without the risk of misalignment, which can cause issues in connection and stability once hitched. When the trailer is raised fully, it also provides sufficient clearance to avoid any interference between the trailer and the tractor's hitch mechanism. This practice aids in creating a secure connection for safe operation thereafter. Maintaining proper height alignment is crucial as it impacts the ease of coupling and the overall safety of the vehicle combination. Therefore, having the trailer fully raised is the correct procedure before backing the tractor under it.