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

An inner orbital complex uses the inner $(n-1)d$ orbitals in hybridisation ($d^2sp^3$); a diamagnetic complex has no unpaired electrons.
$[Co(NH_3)_6]^{3+}$: $Co^{3+}$ is $3d^6$. $NH_3$ pairs up the six electrons in three of the 3d orbitals, leaving two 3d orbitals empty for $d^2sp^3$ hybridisation. So it is an inner orbital, low spin complex with no unpaired electron, i.e. diamagnetic.
$[Zn(NH_3)_6]^{2+}$: $Zn^{2+}$ is $3d^{10}$; no inner d orbital is free, so it would use $sp^3d^2$ (outer orbital), although it is diamagnetic.
$[Ni(NH_3)_6]^{2+}$: $Ni^{2+}$ is $3d^8$; it uses $sp^3d^2$ (outer orbital) hybridisation and has two unpaired electrons, so it is paramagnetic.
$[Cr(NH_3)_6]^{3+}$: $Cr^{3+}$ is $3d^3$; it is an inner orbital ($d^2sp^3$) complex but has three unpaired electrons, so it is paramagnetic.
So the complex that is both inner orbital and diamagnetic is $[Co(NH_3)_6]^{3+}$.