**Core Concept**
The tensile strength of a bone refers to its ability to withstand pulling forces without breaking. This property is crucial for maintaining bone integrity and preventing fractures. Bone tensile strength is primarily attributed to the arrangement and structure of collagen fibers and hydroxyapatite crystals within the bone matrix.

**Why the Correct Answer is Right**
The tensile strength of a bone is mainly due to the presence of collagen fibers, which provide elasticity and flexibility to the bone. When a bone is subjected to tensile forces, the collagen fibers stretch and deform, absorbing the energy and reducing the likelihood of fracture. Additionally, hydroxyapatite crystals, a type of calcium phosphate, contribute to bone hardness and rigidity, further enhancing its tensile strength. The interaction between collagen fibers and hydroxyapatite crystals enables bones to withstand various types of mechanical stresses, including tension.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while bone cells (osteoblasts and osteoclasts) play a crucial role in bone remodeling and repair, they do not directly contribute to bone tensile strength.
**Option B:** This option is incorrect because bone marrow, which contains blood cells and fat cells, does not directly contribute to bone tensile strength. While bone marrow is essential for bone health, its primary function is to produce blood cells.
**Option C:** This option is incorrect because while bone density is an important factor in determining bone strength, it is not the primary contributor to tensile strength. Bone density is influenced by various factors, including age, nutrition, and exercise, but it is not the direct cause of tensile strength.

**Clinical Pearl / High-Yield Fact**
When assessing bone health, clinicians should consider not only bone density but also the quality of the bone matrix, including the arrangement and structure of collagen fibers and hydroxyapatite crystals. This comprehensive approach can help identify patients at risk of fractures and guide targeted interventions to improve bone health.

**Correct Answer: C. Collagen fibers and hydroxyapatite crystals.**