**Core Concept**
The narrowing of the periodontal ligament in labially placed mandibular canines with age is a result of changes in the periodontal tissues due to the physiological process of aging. This process involves the deposition of cementum and bone around the root of the tooth, leading to a reduction in the width of the periodontal ligament.

**Why the Correct Answer is Right**
The correct answer is due to the deposition of cementum and bone around the root of the tooth. Cementum is a thin layer of bone-like tissue covering the root of the tooth, and its deposition is a continuous process throughout life. With age, the rate of cementum deposition increases, leading to a thickening of the cementum layer and a narrowing of the periodontal ligament. This process is accompanied by the deposition of bone around the root, which further contributes to the narrowing of the periodontal ligament. The periodontal ligament is a fibrous connective tissue that attaches the tooth to the surrounding alveolar bone, and its narrowing is a result of the increased thickness of the cementum and bone layers.

**Why Each Wrong Option is Incorrect**
**Option A:** Down growths of gingival epithelium are not responsible for the narrowing of the periodontal ligament. While down growths of gingival epithelium can occur in the form of gingival enlargement, they do not contribute to the narrowing of the periodontal ligament.

**Option B:** Gingival cysts formed from cell rests are a type of developmental anomaly that can occur in the jaw, but they are not responsible for the narrowing of the periodontal ligament. Gingival cysts are small, fluid-filled sacs that form in the gingival tissues, but they do not affect the periodontal ligament.

**Option D:** The reversal of function of horizontal and oblique fibers is not a valid explanation for the narrowing of the periodontal ligament. The periodontal ligament is a complex tissue composed of various types of fibers, including horizontal, oblique, and circular fibers. The function of these fibers is not reversed with age, and their interaction does not contribute to the narrowing of the periodontal ligament.

**Clinical Pearl / High-Yield Fact**
Aging of the periodontal tissues is a continuous process that affects the entire dentition. The deposition of cementum and bone around the root of the tooth is a normal process that occurs throughout life, and it is not limited to labially placed mandibular canines.

**Correct Answer:**
✓ Correct Answer: A. Due to deposition of cementum and bone