The standard reduction potential for Zn^ 2+ / Zn, Ni^ 2+ / Ni and...

The reaction is spontaneous when $\Delta G < 0$ and E_{\text{cell }^{\circ}>0 .\begin{aligned} & \because \Delta G=-n F E^{\circ} \text{ and } E^{\circ}>0 \\ & \text{ Also } E_{\text{cell }^{\circ}=E_{\text{oxi }^{\circ}+E_{\text{red }^{\circ} \text{ (or) } E_{\text{cathode }^{\circ}-E_{\text{anode }^{\circ}\end{aligned}$$Here, Thus,$E_{\text{cell }^{\circ} for (a) -0.21 V , i.e. E_{\text{cell }^0$< 0$ (b) -0.53 V , i.e. E_{\text{cell }^0$< 0$ (c) -0.32 V , i.e. E_{\text{cell }^{\circ} < 0 (d) 0.53 V , i.e. $E^{\circ}>0