A freshly precipitated $\mathrm{Fe}(\mathrm{OH}){}_3$ sol is to be coagulated by adding one of the following sodium salts at the lowest concentration. The best choice is

The $\mathrm{Fe}(\mathrm{OH}){}_3$ sol is positive, so the coagulating ion is the anion. By the Hardy-Schulze rule the power rises with anion valence. $\mathrm{PO}{}_4{}^{3-}$ is trivalent, whereas $\mathrm{Cl}{}^{-}$, $\mathrm{NO}{}_3{}^{-}$ and $\mathrm{Br}{}^{-}$ are monovalent, so $\mathrm{Na}{}_3\mathrm{PO}{}_4$ has the lowest coagulation value.