The concentration of electrolyte required to coagulate a given amount of $\mathrm{As}_{2} \mathrm{S}_{3}$ sol is minimum in the case of

The concentration of electrolyte required to coagulate a given amount of $\mathrm{As}_{2} \mathrm{~S}_{3}$ sol is minimum in case of aluminium nitrate. $\mathrm{As}_{2} \mathrm{~S}_{3}$ is negatively charge sol. In aluminium nitrate, the charge on $\mathrm{Al}$ cation is $+3$ which is maximum among given cations. Hence, its coagulating power is maximum.