Which one of the following is an inner orbital complex as well as diamagnetic in behaviour?

The correct option is [Co(NH3)6]3+. In [Co(NH3)6]3+, the oxidation state of Co is +3 and its coordination number is six. The electronic configuration of 27Co is 1s2, 2s2, 2p6, 3s2, 3p6, 3d7, 4s2. Co3+ = 1s2, 2s2, 2p6, 3s2, 3p6, 3d6. Thus, [Co(NH3)6]3+ shows an inner orbital complex as well as diamagnetic behavior (due to the absence of an unpaired electron). [Zn(NH3)6]2+ → sp3d2 hybridization (outer) and diamagnetic. [Cr(NH3)6]3+ → d2sp3 hybridization (inner) and paramagnetic. Hence, option C is correct.