Correct Answer - C
Polyphosphates like sodium hexametaphosphate (calgon) form soluble complexes with cations like `Ca^(2+)` and `Mg^(2+)` present in hard water and hence used as water softening agents.
`Na_(6)P_(6)O_(18)to 2NA^(+)+Na_(4)P_(6)O_(18)^(2-)`
`M^(2+)+Na_(4)P_(6)O_(18)^(2-)to [Na_(2)MP_(6)O_(18)]^(2-)+2Na^(+)`
