Explanation

(d)
According to Hardy-Schulze rule, coagulating power of ions is directly proportional to charge on ion
$\because Fe(OH)_{3}$is positively charged colloid.
$\therefore$ It will be coagulated by anion.
$(a)KCN - K^{+}\ and\ {CN}^{-}$
$(b){BaCl}_{2} - {Ba}^{2 +}and\ {Cl}^{-}$
$(c)NaCl - {Na}^{+}\ and\ {Cl}^{-}$
$(d){Mg}_{3}\left( {PO}_{4} \right)_{2} - {Mg}^{2 +}and\ {PO}_{4}^{3 -}$
$\because{PO}_{4}^{3 -}$has highest charge among the given ions anions.
$\therefore{Mg}_{3}\left( {PO}_{4} \right)_{2}$ is the most effective in coagulation of $\ Fe(OH)_{3}$sol.