What is the proper action to take when the trailer air supply knob pops out?

Answer: D — Ensure the emergency brakes are activated

When the trailer air supply knob pops out, it indicates a loss of air pressure in the trailer's air brake system. This situation typically results in the automatic application of the trailer's emergency brakes. Therefore, ensuring that the emergency brakes are activated is the critical response, as this action helps to prevent the trailer from rolling away, maintaining safety for both the vehicle and surrounding areas. This response emphasizes the importance of recognizing that when air pressure drops, the system's design safely engages the brakes to stop any potential movement. Maintaining a focus on the activation of emergency brakes ensures that the load remains secure until the issue can be resolved. Other actions, such as checking air pressure or applying parking brakes, do not prioritize immediate safety in the same way and may not address the urgent need to prevent movement following a system failure.