It is a magnitude difference in energy between the two sets of d-orbital i.e. $t_{2g}$ and eg electronic configuration of $d^5$ if $\Delta_0 < P$ then$t_{2g}^3$ and $e_g^2$ 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) $[\mathrm{Fe}(\mathrm{CN})_6]^{3-}=\mathrm{CN}$ is strong field ligand $\mathrm{Fe}=26,\,[\mathrm{Ar}]\,3d^6\,4s^2$ $\mathrm{Fe}^{3+}=3 d^5$ Inner d-complex If strong field ligand is available then $\Delta_0 > P = t_{2g}^5 e_g$ In $[\mathrm{Fe}(\mathrm{F}_6)]^{3-},\,\mathrm{Fe}=[\mathrm{Ar}]\,3d^6\,4s^2$ $\mathrm{Fe}^{3+}=3 d^5$ with a weak field ligand $\Delta_0 < P$ $\because$ so, there is no pairing of electrons in $3d$ orbitals.