(A) Calcium phosphate
Tooth enamel is made up of calcium phosphate, which is the hardest substance in the human body.
This is a standard factual recall question from the Science textbook (Class 10). The source passage mentions that bacteria produce acids which demineralise the enamel (calcium phosphate). Calcium phosphate is the salt that constitutes tooth enamel — remember it as a calcium salt of phosphoric acid. The other options (Mg, Na, Al phosphate) are distractors; none of these form tooth enamel.