In 3d series, the metal having the highest ${\mathrm{M}}^{2+}/\mathrm{M}$ standard electrode potential is

${\mathrm{Cr}}^{+2}/\mathrm{Cr}\to -0.90 \mathrm{V} ; {\mathrm{Fe}}^{+2}/\mathrm{Fe}\to -0.44 \mathrm{V}$

${\mathrm{Cu}}^{+2}/\mathrm{Cu}\to +0.34 \mathrm{V} ; {\mathrm{Zn}}^{+2}/\mathrm{Zn}\to -0.76 \mathrm{V}$

So $\mathrm{Ans}. {\mathrm{Cu}}^{+2}/\mathrm{Cu}$

In case of 3d series of elements, only 'Cu' has +ve electrode Potential because of inability to liberate H₂ from acids.