The recommended concentration of fluoride ion in drinking water is up to 1 ppm as fluoride ion is required to make teeth enamel harder by converting [3Ca$$_3$$(PO$$_4$$)$$_2$$.Ca(OH)$$_2$$] to:

We begin by recalling the actual chemical composition of the chief mineral present in tooth-enamel. Enamel is mainly made of the crystalline substance called hydroxy-apatite, whose formula is

$$\text{Hydroxy-apatite} = 3\,\text{Ca}_3(\text{PO}_4)_2 \cdot \text{Ca(OH)}_2$$

This lattice, although quite hard, still allows a small amount of dissolution in acidic medium because the $$\text{OH}^-$$ ions can be leached out.

Fluoride ions $$\text{F}^-$$ help by replacing these labile hydroxide ions. The general ion-exchange step is stated first:

$$\text{Ca(OH)}_2 + 2\,\text{F}^- \;\longrightarrow\; \text{CaF}_2 + 2\,\text{OH}^-$$

When this exchange is carried out inside the hydroxy-apatite lattice, we simply substitute $$\text{Ca(OH)}_2$$ by $$\text{CaF}_2$$ in the overall crystal formula while keeping the phosphate part unchanged. Therefore

$$3\,\text{Ca}_3(\text{PO}_4)_2 \cdot \underbrace{\text{Ca(OH)}_2}_{\text{present originally}} \;\underset{\;\text{F}^-\,}{\rightarrow}\;\; 3\,\text{Ca}_3(\text{PO}_4)_2 \cdot \underbrace{\text{CaF}_2}_{\text{formed}}$$

The new crystal thus formed is called fluoro-apatite. Because $$\text{CaF}_2$$ is far less soluble than $$\text{Ca(OH)}_2$$, the enamel becomes harder and more resistant to acid attack.

Comparing this result with the options given, we look for the formula containing $$3\,\text{Ca}_3(\text{PO}_4)_2$$ followed by $$\text{CaF}_2$$. That exact expression appears in Option D:

$$[3\text{Ca}_3(\text{PO}_4)_2 \cdot \text{CaF}_2]$$

Hence, the correct answer is Option D.