Explanation

[Solution from source] Ni²⁺ is d⁸; in an octahedral complex the two remaining inner d orbitals are not vacant, so it uses 4s4p³4d² (sp³d², outer orbital) and keeps two unpaired electrons, making it paramagnetic. [Co(NH₃)₆]³⁺ is inner orbital and diamagnetic, [Zn(NH₃)₆]²⁺ is outer orbital but diamagnetic (d¹⁰), and [Cr(NH₃)₆]³⁺ is inner orbital (d²sp³). Answer (2).