Which substance forms the primary matrix of bones?

The correct answer is calcium phosphate, which is the primary inorganic component of bone tissue. Bone is a dynamic structure composed of a matrix that provides both strength and rigidity. Calcium phosphate, specifically in the form of hydroxyapatite crystals, is critical as it makes up around 70% of the bone's weight and contributes to its hardness and structural integrity.

While collagen is indeed important as it provides the organic framework of the bone, serving to give flexibility and tensile strength, the mineral component, including calcium phosphate, is what ultimately gives bone its strength and resistance to compression. Cartilage, on the other hand, is a separate type of connective tissue that can be found in joints but does not constitute the bulk of bone. Fibroblasts are cells that produce collagen and other fibers but are not the primary substance in the matrix of bones. Therefore, acknowledging the distinct roles of these components highlights why calcium phosphate is identified as the primary matrix of bones.