According to Hardy-Scheulze rule, the coagulating power of an electrolyte will be higher for ionic components having greater charges or valencies.
For coagulating a sol of a particular charge, we add particles of opposite charge.
Thus, to coagulate antimony sulphide sol which is negatively charged, we must add an electrolyte with a cation of higher valency.
Thus, $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$ that has $\mathrm{Al}^{3+}$, will be most effective for coagulating antimony sulphide.