Which component is critical to check for safety when coupling a trailer to a tractor?

When coupling a trailer to a tractor, checking the air lines is critical for safety. These air lines are responsible for transmitting air pressure to the trailer's braking system, which is essential for control and stopping ability while driving. A proper connection ensures that the brakes will function correctly, preventing potential accidents caused by brake failure.

In addition to being critical for operational safety, inspecting the air lines allows you to identify any leaks or damage that could result in a loss of pressure or malfunction when the vehicle is in operation. Ensuring that these air lines are properly coupled and in good condition significantly contributes to the overall safety of the combination vehicle.

While checking battery connections, fuel levels, and transmission fluid are also important maintenance tasks, they are not directly related to the safe coupling of a trailer. Therefore, ensuring the integrity of the air lines stands out as the most crucial step in this process.