It is a magnitude difference in energy between the two sets of d-orbital i.e. ${\mathit{t}}_{2\mathit{g}}$ and eg electronic configuration of ${\mathit{d}}^5$ if
then${\mathit{t}}_{2\mathit{g}}^3$ and ${\mathit{e}}_{\mathit{g}}^2$
Because it follows weak field ligand phenomina when weak field ligands are present in a structure pairing of electrons do not take place.
∵ so (b) ${\left[\mathit{F}\mathit{e}{\left(\mathit{C}\mathit{N}\right)}_6\right]}^{3-}=\mathit{C}\mathit{N}$ is strong field ligand
$\mathit{F}\mathit{e}=26,\left[\mathit{A}\mathit{r}\right]3{\mathit{d}}^{6}4{\mathit{s}}^2$
${\mathit{F}\mathit{e}}^{3+}=3{\mathit{d}}^5$
Inner d-complex
If strong field ligand is available then
${\mathrm{\Delta }}_0>\mathit{P}={\mathit{t}}_{2\mathit{g}}^5\mathrm{}{\mathit{e}}_{\mathit{g}}$
In ${\left[\mathit{F}\mathit{e}\left({\mathit{F}}_6\right)\right]}^{3-},\mathit{F}\mathit{e}=\left[\mathit{A}\mathit{r}\right]3{\mathit{d}}^{6}4{\mathit{s}}^2$
${\mathit{F}\mathit{e}}^{3+}=3{\mathit{d}}^5$
with a weak field ligand
$\because$ so, there is no pairing of electrons in $3\mathit{d}$ orbitals.