How can you check if the trailer brakes are functioning properly after supplying air?

The correct option involves applying and releasing the trailer brakes to check for proper functionality after supplying air. This method allows you to confirm that the air is reaching the brakes and that they are responding as expected. When you apply the brakes, you should feel resistance indicating that the brakes are engaging. Upon releasing, the brakes should disengage smoothly, confirming that they are functioning properly.

This technique is crucial in ensuring safe operation, especially in combination vehicles where both the tractor and trailer brakes need to work seamlessly together. Observing the brake lights is not a sufficient test, as it does not confirm the actual braking system's functionality. Smelling for burning brakes could indicate a problem but is not a proactive method for checking brake operation. Lastly, checking the brake fluid level is more relevant for hydraulic brake systems rather than air-brake systems typically found in combination vehicles.