The S-cam:
Correct. When the foot valve is pressed, air pushes the brake-chamber pushrod, which moves the slack adjuster, which rotates the S-cam shaft, forcing the shoes against the drum.
CDL Practice Question #102
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.