The presence of soluble fluoride ion upto 1 ppm concentration in drinking water, is:

We begin by recalling the meaning of the unit “ppm”. The abbreviation “ppm” stands for “parts per million”. Quantitatively, $$1\ \text{ppm} = \frac{1\ \text{part of solute}}{10^{6}\ \text{parts of solution}}.$$ When applied to water, this means $$1\ \text{ppm} = 1\ \text{mg of solute per litre of water},$$ because one litre of water has a mass very close to $$10^{6}\ \text{mg}$$.

Now we consider fluoride ions $$\mathrm{F^-}$$ in drinking water. A very small amount of fluoride is known to interact with the surface of tooth enamel. Enamel is primarily composed of the mineral hydroxy-apatite, whose chemical formula is $$\mathrm{Ca_5(PO_4)_3OH}.$$ Fluoride ions can replace the hydroxide ion $$\mathrm{OH^-}$$ in this lattice to produce fluoro-apatite, $$\mathrm{Ca_5(PO_4)_3F}.$$

Fluoro-apatite is chemically less soluble in acids than hydroxy-apatite. Therefore, when a limited quantity of fluoride (about $$1\ \text{ppm}$$) is present in water, it strengthens the enamel and makes teeth more resistant to decay caused by bacterial acids. This phenomenon is the scientific basis of community water fluoridation programmes carried out around the world.

We must also remember that only a narrow range of fluoride concentration is beneficial. If the concentration rises significantly above $$1\ \text{ppm}$$ (for example, to $$3\text{-}4\ \text{ppm}$$ or higher), chronic consumption can lead to dental fluorosis (mottling and discolouration of teeth) and at still higher levels to skeletal fluorosis, which affects bones. However, the question specifically states “up to $$1\ \text{ppm}$$”, i.e. at or below the optimal protective level.

Therefore, within the given limit of $$1\ \text{ppm}$$, the presence of soluble fluoride ion is not harmful; on the contrary, it is beneficial because it protects teeth from decay.

Among the options, the statement that matches this conclusion is “safe for teeth”.

Hence, the correct answer is Option D.