Polyphosphates like sodium hexametaphosphate (calgon) are used as water softening agents because they:

The correct option is C form soluble complexes with cationic species
Polyphosphates like sodium hexametaphosphate (calgon) form souble complex with cations like $C{a}^{2+}$ and $M{g}^{2+}$present in hard water and hence are used as water softening agents.
$N{a}_6{P}_6{O}_{18}\to 2N{a}^{+}+N{a}_4{P}_6{O}_{18}^{2-}$
$M^{2+}+N{a}_4{P}_6{O}_{18}^{2-}\to [N{a}_{2}M{P}_6{O}_{18}{]}^{2-}+2N{a}^{+}$