What function does the trailer air supply control serve?

The trailer air supply control plays a crucial role in managing the air supply to the trailer's braking system. Its primary function is to supply compressed air to the trailer when it is connected to a tractor and also to shut off the air supply when the trailer is disconnected. This is essential for operating the trailer's brakes effectively and ensuring safety during transport. Proper air supply allows the brakes to function properly, which is vital for maintaining control of the trailer while being towed.

The other options do not accurately reflect the function of the trailer air supply control. Adjusting air pressure in tires relates to tire maintenance, while controlling engine temperature deals with the vehicle's engine cooling system. Increasing speed on downhill is more about vehicle dynamics and does not involve air supply control mechanisms. Thus, the emphasis on the management of air supply highlights why the correct choice is the one that addresses this critical function in operating a trailer safely.