**Core Concept**
Reid's index is a measure of the ratio of the diameter of the airway to the diameter of the surrounding tissue, used to assess airway wall thickening and obstruction. It is particularly relevant in chronic respiratory diseases where airway remodeling occurs.

**Why the Correct Answer is Right**
In chronic bronchitis, there is chronic inflammation leading to increased mucus production, airway wall thickening, and smooth muscle hypertrophy. This results in a significant increase in the Reid's index due to the disproportionate growth of the airway wall relative to the lumen. The index reflects structural changes seen in long-standing bronchial inflammation, which is a hallmark of chronic bronchitis.

**Why Each Wrong Option is Incorrect**
Option A: Bronchial asthma does not show consistent airway wall thickening; it presents with reversible obstruction and variable airway narrowing, not a fixed increase in Reid's index.
Option B: Bronchiectasis involves dilatation of airways and loss of normal airway structure, but the Reid's index is not typically increased due to wall thickening; instead, it shows dilatation with wall thinning.
Option D: Pneumonia is an acute infection causing localized inflammation and consolidation, not chronic airway remodeling; thus, Reid's index remains normal or is not significantly altered.

**Clinical Pearl / High-Yield Fact**
Reid's index is most useful in chronic bronchitis to detect irreversible airway obstruction due to structural changes. Remember: increased Reid's index = chronic bronchitis; decreased or normal = asthma or pneumonia.

✓ Correct Answer: C. Chronic bronchitis