Cartilage provides structural support in larger airways

Steps:

• Identify the gas exchange system's structures: bronchi, bronchioles, alveoli, and capillaries.
• Recall that cartilage maintains airway openness against collapse during breathing.
• Check each option for cartilage presence: bronchi have it, others do not.
• Confirm via anatomy: only bronchi universally contain cartilage rings.

Why D is correct:

• Bronchi are larger airways reinforced by C-shaped hyaline cartilage rings to prevent collapse and ensure airflow.

Why the others are wrong:

• A. Alveoli are thin, elastic air sacs for gas diffusion, lacking any cartilage.
• B. Bronchioles are narrow tubules without cartilage, relying on smooth muscle.
• C. Capillaries are microscopic blood vessels for exchange, composed of endothelium only.

Final answer: D