Which of the following ion configurations has zero $CFSE$ in both strong and weak field ligands in an octahedral complex?

What is crystal field splitting energy? How does the magnitude of $\Delta_{o}$ decide the actual configuration of $d$-orbitals in a coordination entity?

$[Fe(CN)_{6}]^{3-}$ is an inner orbital complex. Ignoring the pairing energy,the value of crystal field stabilization energy for this complex is $(-)\;\dots \;\Delta_{o}$.

For a $d^{4}$ metal ion in an octahedral field,the correct electronic configuration is:

$A$ $[M(H_2O)_6]^{2+}$ complex typically absorbs at around $600 \ nm$. It is allowed to react with ammonia to form a new complex $[M(NH_3)_6]^{2+}$ that should have absorption at ....... $nm$.

The crystal field splitting energy for octahedral $\Delta _0$ and tetrahedral $\Delta _t$ complexes is related as