Tracheal smooth muscle adjusts airway diameter

Steps:

• Identify the trachea's structure: outer cartilage rings provide support, inner lining includes epithelium and submucosa.
• Recognize that diameter adjustment requires contractile tissue, specifically smooth muscle, located posteriorly between cartilage ends.
• Examine labeled sections: A likely epithelium, B cartilage, C submucosa or glands, D posterior smooth muscle band.
• Confirm D as the tissue capable of contraction/relaxation via autonomic innervation.

Why D is correct:

• Smooth muscle contracts via actin-myosin interactions, narrowing airways during expiration or irritation, as defined in respiratory physiology.

Why the others are wrong:

• A: Epithelium protects and secretes mucus but lacks contractility.
• B: Cartilage provides rigid support and cannot contract.
• C: Submucosa contains glands for lubrication, not muscle fibers.

Final answer: D