**Core Concept**
The underlying principle being tested is the composition of **hydroxyapatite**, a key mineral found in bone tissue. Hydroxyapatite is a form of **calcium apatite**, crucial for the structure and strength of bones. Its formula reflects its chemical composition.

**Why the Correct Answer is Right**
The correct formula for hydroxyapatite is , indicating it is composed of calcium, phosphate, and hydroxide ions. This formula is essential for understanding the chemical structure of bone and how it contributes to bone strength and density. The **calcium** and **phosphate** ions are crucial for the formation of the hydroxyapatite crystal lattice.

**Why Each Wrong Option is Incorrect**
**Option A:** Is incorrect because it does not accurately represent the ratio of calcium to phosphate to hydroxide in hydroxyapatite.
**Option B:** Is incorrect as it misrepresents the chemical composition of hydroxyapatite.
**Option C:** Is incorrect because it does not correctly formulate the hydroxyapatite crystal.

**Clinical Pearl / High-Yield Fact**
Remembering the formula of hydroxyapatite is crucial for understanding bone pathology and diseases related to bone mineralization, such as **osteoporosis**. The composition of hydroxyapatite is key to understanding how bones are formed and maintained.

**Correct Answer:** D. Ca₁₀(PO₄)₆(OH)₂