Answer:
Option A
Explanation:
Calgon is a trading name of a complex salt, sodium hexametaphosphate $(NaPO_{3})_{6}$ . Calgon ionises to give a complex anion. The addition of Calgon to hard water to causes the calcium ions of hard water to displace sodium ions from the anion of Calgon and form a complex with Calgon.
$(NaPO_{3})_{6} $ or
$Na_{2}(Na_{4}P_{6}O_{18})\rightarrow 2Na^{+}+Na_{4}P_{6}O_{18}^{2-}$(complex anion)
$Ca^{2+}$(from hard water)+ $ Na_{4}P_{6} O_{18}^{2-}$( Anion of calgon) $\rightarrow$ $2 Na^{+}+ CaNa_{2}P_{6}O_{18}^{2-}$ ( Goes into solution)
