The electrode potential of $$M^{2+}/M$$ of 3d-series elements shows positive value for?

We need to find which 3d-series element has a positive electrode potential for the $$M^{2+}/M$$ couple.

The standard electrode potential $$E^{\circ}(M^{2+}/M)$$ for 3d transition metals is generally negative because these metals tend to lose electrons easily and go into solution. However, copper is the exception.

The standard reduction potential values are approximately: $$E^{\circ}(Fe^{2+}/Fe) = -0.44 \text{ V}$$, $$E^{\circ}(Co^{2+}/Co) = -0.28 \text{ V}$$, $$E^{\circ}(Zn^{2+}/Zn) = -0.76 \text{ V}$$, and $$E^{\circ}(Cu^{2+}/Cu) = +0.34 \text{ V}$$.

Copper has a positive $$E^{\circ}$$ value because of its high enthalpy of atomisation and relatively high ionisation enthalpy combined with low hydration enthalpy. This means copper does not easily lose electrons to go into solution.

Hence, the correct answer is Option D.