Explanation

(d)
${As}_{2}S_{3}$sol is a negative sol due to preferential adsorption of $S^{2 -}ions\ \left\lbrack {As}_{2}S_{3} \right\rbrack S^{2 -}$. Hence, a cation is needed to coagulate it. Accodig to Hardy-Schulze rule, aluminium nitrate will be the most efficient to coagulate it, as it gives the most valent aluminium ion $\left( {Al}^{3 +} \right),$ hence it is required in minimum amount. (i.e.,coagulation value is least for $Al\left( {NO}_{3} \right)_{3}.$