Concept:Tooth enamel is the hardest substance in the human body and is primarily composed of calcium phosphate in the form of hydroxyapatite crystals.Explanation:Enamel is about 96% inorganic minerals, mainly calcium phosphate. This mineral gives enamel its strength and resistance to wear. It also protects teeth from acidic damage. Unlike other body tissues, enamel is non-living and cannot regenerate if damaged. Proper oral hygiene and fluoride help maintain calcium phosphate levels. Other options: Sodium hydrogen carbonate (baking soda) is used in toothpaste to neutralize acids but is not a structural component. Sodium hydroxide (caustic soda) is a corrosive industrial chemical, not present in enamel. Lactic acid is produced by bacteria and causes enamel demineralization, leading to cavities.Answer:A. Calcium phosphate