What action is performed to apply the service brakes on a trailer?

Applying the hand control, also known as the trolley valve, is the correct action to engage the service brakes on a trailer. This control allows the driver to manually apply the trailer brakes independently from the tractor brakes. It is especially useful when controlling the braking of the trailer in various driving conditions, such as during a sharp turn, when descending a hill, or when needing to stabilize the vehicle during an emergency stop.

The hand control provides a way for the driver to increase control over brake application, allowing for smoother stops and better handling of the trailer. This is crucial for maintaining safety, particularly when towing heavy loads where the trailer could influence the overall vehicle dynamics.

In contrast, adjusting the brake ratio affects how the brakes are distributed between the tractor and trailer, which is not the immediate action required to apply brakes. Pulling the air supply control is related to the initial engagement of the trailer's air supply system rather than directly applying the brakes. Inspecting the brake lights pertains to ensuring that the lights are functional rather than actively engaging the braking system itself.