The correct option is (B) [Co(H2O)6]2.

Explanation:

Magnetic moment is a measure of the strength of the magnetic field around a substance. It arises due to the presence of unpaired electrons in the substance. The unpaired electrons can be in partially filled orbitals, which can interact with an external magnetic field.

The complex that is expected to show orbital contribution to the overall magnetic moment is the one that has unpaired electrons in partially filled orbitals. Among the given options, the complex [Co(H2O)6]2 is expected to show such contribution. This is because cobalt has the electronic configuration [Ar] 3d7 4s2, where the 3d orbitals are partially filled. The six water molecules in the complex donate their lone pairs of electrons to form coordination bonds with the cobalt ion, which results in the formation of an octahedral complex with six ligands. The electrons in the partially filled 3d orbitals of the cobalt ion can interact with the external magnetic field, resulting in the overall magnetic moment of the complex.

On the other hand, the other complexes given in the options do not have unpaired electrons in partially filled orbitals, and hence, they are not expected to show such contribution to the overall magnetic moment. For instance:

- [Cr(CN)6]3- has a d3 electronic configuration, where all the d orbitals are completely filled with electrons in the presence of six cyanide ligands. Hence, there are no unpaired electrons to contribute to the magnetic moment.
- [Ni(en)3]2+ has a d8 electronic configuration, where all the d orbitals are completely filled with electrons in the presence of three bidentate ethylenediamine ligands. Hence, there are no unpaired electrons to contribute to the magnetic moment.
- [Cu(NH3)6]2+ has a d9 electronic configuration, where all the d orbitals are completely filled with electrons in the presence of six ammonia ligands. Hence, there are no unpaired electrons to contribute to the magnetic moment.

Therefore, the correct option is (B) [Co(H2O)6]2.