The airwaykidneybrain diaphragm is the muscle that expands and contracts, allowing air to fill the chest cavity.

The diaphragm is the muscle beneath the lungs that contracts and relaxes to help move air into and out of the chest. “Airway,” “kidney,” and “brain” are not the name of this breathing muscle, so the correct choice is diaphragm.

AI-written and checked against the verified answer.