What does the term "air supply knob" refer to in testing the tractor protection valve?

The term "air supply knob" specifically refers to a knob that supplies air to the trailer. When testing the tractor protection valve, the function of this knob is crucial because it allows the tractor's air supply to be directed to the trailer's air systems.

In practice, this means when you turn the air supply knob, you are enabling air to flow from the tractor to the trailer, which is essential for the operation of the trailer's braking system and other pneumatic functions. This function is especially vital during pre-trip inspections to ensure that the air supply is functioning correctly and that there are no leaks between the tractor and the trailer.

The other options refer to different functions that do not align with the specific role of the air supply knob. Thus, the correct answer accurately captures the functionality intended with the term.