The trailer air-supply control:
Correct. The trailer air-supply (tractor protection) is a red eight-sided knob. Push in to supply air to the trailer; pull out to shut off air.
CDL Practice Question #111
Category: Air Brakes
Why this question matters
This question is one of 71 in our Air Brakes bank. The reasoning above comes directly from the AAMVA CDL Manual, the source document used by every U.S. state DMV. Memorizing the underlying rule — not just the letter answer — is what separates passing on the first attempt from a return trip and another fee.