How is the $t_{2g}^4 e_g^0$ configuration possible for a $d^4$ ion during crystal field splitting in an octahedral complex?

Consider the metal complexes $[Ni(en)_3]^{2+}$ $(A)$, $[NiCl_4]^{2-}$ $(B)$ and $[Ni(NH_3)_6]^{2+}$ $(C)$. Choose the $\text{CORRECT}$ option by considering the number of unpaired electrons present in $(A)$, $(B)$ and $(C)$ respectively and the order of frequency of absorption.

On the basis of crystal field theory,explain why $Co(III)$ forms a paramagnetic octahedral complex with weak field ligands,whereas it forms a diamagnetic octahedral complex with strong field ligands.

$[Fe(H_2O)_6]^{3+}$ is strongly paramagnetic whereas $[Fe(CN)_6]^{3-}$ is weakly paramagnetic. Explain.

Given below are two statements $:$
Statement $(I) :$ In octahedral complexes,when $\Delta_{o} < P$ high spin complexes are formed. When $\Delta_{o} > P$ low spin complexes are formed.
Statement $(II) :$ In tetrahedral complexes because of $\Delta_{t} < P$,low spin complexes are rarely formed.
In the light of the above statements,choose the most appropriate answer from the options given below $:$

Calculate the crystal field stabilization energy $(CFSE)$ in $[Co(CN)_6]^{3-}$.