**Core Concept**
Metaplasia is a physiological or pathological process where one type of adult cell is replaced by another type of adult cell, often in response to environmental or pathological stimuli. This process is characterized by the replacement of a mature cell type with a different mature cell type, which may be more resistant to injury or disease.

**Why the Correct Answer is Right**
Metaplasia is a reversible change that occurs in response to chronic irritation or injury, where the original cell type is replaced by a more resilient cell type. For example, in the case of chronic bronchitis, the ciliated columnar epithelial cells of the bronchial lining are replaced by squamous epithelial cells, which are more resistant to the chronic irritation caused by cigarette smoke. This process is often seen in response to environmental exposures, such as smoking or asbestos exposure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option describes dysplasia, which is a disordered or abnormal growth of cells, often associated with neoplastic changes.
**Option B:** This option describes atrophy, which is a reduction in cell size or number, often due to decreased workload or disease.

**Clinical Pearl / High-Yield Fact**
Metaplasia is a reversible change that can be a precursor to more serious conditions, such as carcinoma. For example, squamous metaplasia in the bronchial lining can progress to squamous cell carcinoma.

**✓ Correct Answer: C. Replacement of one type of adult cell by another type of adult cell**