form soluble complexes with cationic species
Polyphosphates are used as water softening agents because they form soluble complexes with cationic species of hard water, which are responsible for the hardness of the water.
The hardness of the water is mainly caused by the presence of naturally occurring dissolved calcium and magnesium. Here is a chemical reaction of the water softening process in the presence of polyphosphates-A water softening agents
2CaSO4 + Na2[Na4(PO3)6 → Na2 [Ca2(PO3)6] + 2Na2SO4
2MgSO4 + Na2[Na4(PO3)6]→ Na2[Mg2(PO3)6 + 2Na2SO4
So, the correct option is (A)