Explanation

(d)
Ferric hydroxide sol is positively charged sol. It is coagulated by negative ions. Larger the charge on anion, larger is its coagulating power or smaller is its flocculation value. In $K_{4}\left\lbrack Fe(CN)_{6} \right\rbrack$, the anion $\left\lbrack Fe(CN)_{6} \right\rbrack^{4 -}$ has highest charge, therefore $K_{4}\lbrack Fe(CN)_{6}\rbrack$ is most effective in coagulating $Fe(OH)_{3}$ sol.