A reaction is feasible if EMF of the cell reaction is positive. (i) $\mathrm{Fe}^{3+}_{(aq)} + \mathrm{I}^{-}_{(aq)} \rightarrow \mathrm{Fe}^{2+}_{(aq)} + \frac{1}{2} \mathrm{I}_2$, $\text{i.e., } \mathrm{Pt}|\mathrm{I}_2|\mathrm{I}^{-}_{(aq)}||\mathrm{Fe}^{3+}_{(aq)}|\mathrm{Fe}^{2+}_{(aq)}|\mathrm{Pt}$ $\therefore E^\circ_{\text{cell}} = E^\circ_{\mathrm{Fe}^{3+}/\mathrm{Fe}^{2+}} - E^\circ_{\frac{1}{2}\mathrm{I}_2/\mathrm{I}^{-}}$ $= 0.77 - 0.54 = 0.23\,\text{V}($Feasible$)$ $\text{i.e., } \mathrm{Cu}|\mathrm{Cu}^{2+}_{(aq)}||\mathrm{Ag}^{+}_{(aq)}|\mathrm{Ag}$ $E^\circ_{\text{cell}} = E^\circ_{\mathrm{Ag}^{+}/\mathrm{Ag}} - E^\circ_{\mathrm{Cu}^{2+}/\mathrm{Cu}}$ $= 0.80 - 0.34 = 0.46\,\text{V}($Feasible$)$ $\text{i.e., } \mathrm{Br}_2|\mathrm{Br}^{-}_{(aq)}||\mathrm{Fe}^{3+}_{(aq)}|\mathrm{Fe}^{2+}_{(aq)}$ $E^\circ_{\text{cell}} = E^\circ_{\mathrm{Fe}^{3+}/\mathrm{Fe}^{2+}} - E^\circ_{\frac{1}{2}\mathrm{Br}_2/\mathrm{Br}^{-}}$ $= 0.77 - 1.09 = -0.32\,\text{V}$(Not feasible) $\text{i.e., } \mathrm{Ag}_{(s)}|\mathrm{Ag}^{+}_{(aq)}||\mathrm{Ag}^{3+}_{(aq)}|\mathrm{Ag}^{2+}_{(aq)}$ $E^\circ_{\text{cell}} = E^\circ_{\mathrm{Fe}^{3+}/\mathrm{Fe}^{2+}} - E^\circ_{\mathrm{Ag}^{+}/\mathrm{Ag}}$ $= 0.77 - 0.80 = -0.03\,\text{V}$ (Not feasible) $\text{i.e., } \mathrm{Fe}^{2+}_{(aq)}|\mathrm{Fe}^{3+}_{(aq)}||\mathrm{Br}_2|\mathrm{Br}^{-}$$E^\circ_{\text{cell}} = E^\circ_{\frac{1}{2}\mathrm{Br}_2/\mathrm{Br}^{-}} - E^\circ_{\mathrm{Fe}^{3+}/\mathrm{Fe}^{2+}}$$= 1.09 - 0.77 = 0.32\,\text{V}$ (Feasible)