Permanent hardness of water can be removed by:
(a) Heating
(b) Treating with sodium acetate $(C{H_2}C{O_2}Na)$
(c) Treating with $Ca{\left( {HC{O_3}} \right)_2}$
(d) Treatment with sodium hexametaphosphate $(N{a_6}{P_6}{O_{18}})$

Answer Verified Verified
Hint: Try to recall that permanent hardness of water is due to the presence of soluble chlorides and sulphates of magnesium and calcium. Now by using this you can easily find the most appropriate option for the given question.

Complete step by step answer:
It is known to you that water which does not produce lather with soap readily is called hard water.
Hardness of water is of two types:
1-Temporary hardness (can be removed by boiling)
2-Permanent hardness (cannot be removed by simple boiling).
Permanent hardness of water is due the presence of chlorides and sulphates of calcium and magnesium. This type of hardness is called permanent hardness since it cannot be removed by simply boiling the water.
There are various methods by which permanent hardness of water can be removed but here we will look at the Calgon process.
Calgon process:In this process, $C{a^{2 + }}$ and $M{g^{2 + }}$ present in hard water are removed by the treatment with sodium hexametaphosphate $(N{a_6}{P_6}{O_{18}})$. The trade name of sodium hexametaphosphate is Calgon (which means calcium gone). Therefore, from above we can conclude that option D is the correct option to the given question.

Note: It should be remembered that when Calgon is added to hard water, then $C{a^{2 + }}$ and $M{g^{2 + }}$ present in it combine with sodium hexametaphosphate to form soluble complex of calcium and magnesium salts.
Calgon is also used in synthetic detergents to check the wastage of soap during washing of clothes.