Which of the following complexes has the highest value of crystal field splitting energy $(\Delta_o)$?

The electron distribution in $d^{n}$ coordination complexes depends on the magnitude of crystal field splitting,$(\Delta_{0})$ and pairing energy $(P).$ The condition which favours the formation of high spin complexes is

For which of the following pairs of complexes is the crystal field splitting energy $\Delta_0$ for $II > \Delta_0$ for $I$?

$I$	$II$
$(a). [Cr(H_2O)_6]^{2+}$	$[Cr(H_2O)_6]^{3+}$
$(b). [Fe(H_2O)_6]^{3+}$	$[Fe(CN)_6]^{3-}$
$(c). [Fe(CN)_6]^{3-}$	$[Ru(CN)_6]^{3-}$
$(d). [NiF_6]^{4-}$	$[NiF_6]^{2-}$

In which of the following complexes of the $Co$ (at. no. $27$),will the magnitude of $\Delta_o$ be the highest?

What is pairing energy $(P)$? Explain the distribution of $d^n$ ($n=1$ to $10$) electrons in high spin and low spin octahedral complexes.

Given below are two statements: Statement $I$: Each electron in $e_g$ orbitals destabilizes the orbitals by $+0.6\Delta_o$ and each electron in the $t_{2g}$ orbitals stabilizes the orbitals by $-0.4\Delta_o$ in an octahedral field on the basis of crystal field theory. Statement $II$: All the $d$-orbitals of the transition metals have the same energy in their free atomic state but when a complex is formed the ligands destroy the degeneracy of these orbitals on the basis of crystal field theory. In the light of the above statements, choose the correct answer from the options given below: