Cartilage supports airways in gas exchange system

Steps:

• Examine photomicrograph for structural features in respiratory tract sections.
• Identify rigid, ring-like or plate-like structures maintaining airway patency.
• Distinguish from epithelial linings or muscle layers by density and location.
• Match to cartilage, common in trachea and bronchi.

Why D is correct:

• Cartilage provides structural support in trachea and larger bronchioles, appearing as dense, bluish rings in histological sections per standard anatomy.

Why the others are wrong:

• A: Ciliated epithelium lines airways for mucus clearance but lacks rigid support.
• B: Smooth muscle regulates bronchoconstriction, appearing as thin layers, not prominent rings.
• C: Squamous epithelium forms alveoli for diffusion, thin and flat, not supportive.

Final answer: D