Which of the following cations will have minimum flocculation value for the arsenious sulphide sol?

The coagulation or flocculation value of an electrolyte is the amount of the electrolyte that must be added to one litre of a colloidal solution in order to fully coagulate the solution. The amount of electrolyte needed to coagulate a specific amount of coagulating ion decreases as the valency of the coagulating or flocculating ion increases. Thus, trivalent cations $\left(A{l}^{3+}\right)$ are much more effective than divalent $\left(B{a}^{2+}\right)$ cations, which in turn are more successful than monovalent $\left({\mathrm{Na}}^{+}\right)$ cations for coagulation of negatively charged arsenious sulphide solution. So, the arsenic sulphide solution's flocculating value is in the following order: $N{a}^{+}>{\mathrm{Ca}}^{2+}=M{g}^{2+}>A{l}^{3+}$