ANS:B - calcium phosphate.

The main constituent of bone ash is calcium phosphate. Bone ash is produced by calcining (heating to high temperatures) animal bones, typically from cattle, to remove organic matter and water, leaving behind a white, powdery residue known as bone ash. The principal component of bone ash is calcium phosphate, which accounts for a significant portion of the bone's mineral content. Calcium phosphate is a mineral compound found naturally in bones and teeth, where it provides strength and rigidity. In bone ash, calcium phosphate remains as the primary mineral constituent even after the organic components have been removed through calcination. Therefore, the correct answer is calcium phosphate.