You supply air to the trailer tanks by:

Pushing in the trailer air supply valve.. Section 6.2: Pushing the red knob IN supplies air.

You supply air to the trailer tanks by:

ANSWER BREAKDOWN
  • A. Correct. Pushing the red knob IN supplies air.
  • B. Incorrect. "Pulling out the trailer air supply valve." is not correct. Pushing the red knob IN supplies air.
  • C. Incorrect. "Stepping on the brake." is not correct. Pushing the red knob IN supplies air.
  • D. Incorrect. "Pulling the hand valve." is not correct. Pushing the red knob IN supplies air.