Polyphosphates are used for softening agents because they

Answer

D

Explanation

Polyphosphate (e.g., sodium hexa metaphosphate) form soluble complexes with cations such as `Ca^(21), Mg^(21)` and so they are used as water softening agents.
`underset("Sod, hexa metaphosphate")underset("..Calgon..")(Na_(2)[Na_(4)(PO_(3))_(6)])+underset("Hardness")(2Ca^(2+))rarr underset("..Soluble..")(Na_(2)[Ca_(2)(PO_(3))_(6)])+4Na^(+)`