**Core Concept**
Cementum is a specialized bone-like tissue covering the root of a tooth, playing a crucial role in tooth anchorage and periodontal ligament attachment. Fully mineralized cementum exhibits unique mechanical properties, which are essential for its function in the oral environment.

**Why the Correct Answer is Right**
Fully mineralized cementum has a hardness comparable to that of dentin, due to its high concentration of hydroxyapatite crystals. This hardness is attributed to the presence of a high degree of mineralization, which is achieved through the activity of cementoblasts and cementoclasts. The mineralization process involves the deposition of calcium and phosphate ions into a collagen matrix, resulting in the formation of a hard, calcified tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as cementum is not as hard as enamel, which has the highest hardness among all tooth structures.
**Option B:** This option is incorrect as cementum is not as soft as periodontal ligament, which is a fibrous connective tissue.
**Option C:** This option is incorrect as cementum does not have a hardness comparable to that of bone, which has a different mineral composition and structure.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that cementum is a dynamic tissue that undergoes continuous turnover, with cementoblasts and cementoclasts playing a crucial role in its formation and resorption.

**Correct Answer: D.** Fully mineralized cementum has a hardness comparable to that of dentin.