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

We need to find the 3d series metal with the highest (most positive) $$M^{2+}/M$$ standard electrode potential. The standard reduction potentials $$E°(M^{2+}/M)$$ for some 3d metals are:

$$Cr: -0.91$$ V
$$Fe: -0.44$$ V
$$Cu: +0.34$$ V
$$Zn: -0.76$$ V

Among these, copper (Cu) has the highest (most positive) standard electrode potential at $$+0.34$$ V. In fact, Cu is the only 3d metal with a positive $$E°(M^{2+}/M)$$ value, which means $$Cu^{2+}$$ ions are relatively easy to reduce to Cu metal.

This is because copper has high atomization enthalpy, high ionization enthalpy, and low hydration enthalpy for $$Cu^{2+}$$, making the overall reduction favorable.

The answer is $$\boxed{\text{Option C: Cu}}$$.