Among the following electrolytes which is the most effective coagulating agent for $\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$ sol.

According to Hardy-Schulze rule, coagulation becomes more effective on the addition of the higher opposite charged electrolyte in the solution. $\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$ is a positive charged sole and ${\mathrm{Na}}_3{\mathrm{PO}}_4$ is known to be the effective coagulation agent as it has a greater $–\mathrm{ve}$ charge because of having ${\mathrm{PO}}_4^{3-}$ that has $– 3$ charge.