Which device measures the pressure in the air tanks?

Answer: C — The supply pressure gauge

Measuring the air in the tanks is what this item tests. The device that shows how much air is stored in the tanks is the supply pressure gauge. It displays the current pressure in the air reservoir system, usually in psi, so you can confirm there’s enough pressure to safely operate the brakes and other air-powered systems. The brake pedal is used to apply braking force, but it doesn’t measure pressure. The tractor protection valve is a safety device that isolates air to the trailer if the supply drops; it doesn’t provide a pressure reading. The wheel cylinder is part of the braking mechanism that uses air pressure to push the brake shoes, not a measuring instrument.