**Core Concept:** Bone matrix is the primary structural and mechanical support of bones, composed of inorganic substances, organic substances, and cells. The inorganic substances are mainly hydroxyapatite crystals, which make up the mineral content. The organic substances are collagen fibers and various proteins that provide strength and shape to the bone matrix. Osteocytes, osteoblasts, and osteoclasts are the bone cells involved in bone remodeling and maintenance.

**Why the Correct Answer is Right:** Hydroxyapatite is the primary mineral component of bone matrix, making up about 70-80% of its mass. It is a calcium phosphate crystal that is essential for bone strength and mineralization. The correct answer (D) includes hydroxyapatite as one of the components of bone matrix, indicating its crucial role in bone structure and function.

**Why Each Wrong Option is Incorrect:**

A. Hydroxyapatite is not a correct option because it is a part of the correct answer (D).
B. Collagen is an organic component of bone matrix, but it is not a mineral crystal. Collagen provides the organic framework for hydroxyapatite to be deposited and maintained within the bone matrix.
C. Osteocytes, osteoblasts, and osteoclasts are bone cells involved in bone remodeling and maintenance, not mineral crystals.

**Why It Matters:** Understanding the composition of bone matrix is crucial for understanding bone health, diagnosis, and treatment. Knowledge of hydroxyapatite and collagen helps medical professionals to evaluate patients with bone-related issues, such as osteoporosis, osteogenesis imperfecta, or fractures. Additionally, it contributes to the understanding of bone healing, remodeling, and the role of cells in maintaining bone health.