What should be done to ensure the trailer emergency brakes are functioning?

To ensure the trailer emergency brakes are functioning, charging the trailer air system and observing that the trailer rolls freely demonstrates that the brakes are disengaging properly when they should be. This process is crucial because it verifies that the air supply to the trailer is adequate, allowing the brakes to release. If the trailer rolls freely, it indicates that there is no residual pressure causing the brakes to stick, which is essential for safe operation and to confirm that the emergency brakes will engage when needed.

The other options focus on different aspects of trailer operation that do not directly address the testing of emergency brake functionality. For instance, pulling out the normal brake control relates to activating the standard brakes rather than assessing the emergency system. Checking the trailer height before coupling is important for safe connection between the tractor and trailer but does not verify brakes. Engaging the hand control brakes is another way to engage the brakes, but it does not encompass the step of ensuring the emergency system is functioning independently by checking for a free roll once the air system is charged.