Explanation

(c)
Negative colloid is coagulated by positive ion or vice-versa. Greater the valency of coagulating ion, greater will be coagulating ion, greater will be coagulating power.
$(a){ZnSO}_{4} \rightarrow {Zn}^{2 +} + {SO}_{4}^{2 -}$
$(b){Na}_{3}{PO}_{4} \rightarrow 3{Na}^{+} + {PO}_{4}^{3 -}$
$(c){AlCl}_{3} \rightarrow {Al}^{3 +} + 3{Cl}^{-}$
$(d)K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack \rightarrow 4K^{+} + \left\lbrack Fe(CN)_{6} \right\rbrack^{4 -}$
Since, in AlCl_{3}, the valency of positive ion (coagulation ion) is highest, it is the most powerful coagulating agent among the given to coagulate the negative colloid.