Which of the following complexes will show the largest splitting of $d$-orbitals?

Which complex shows the highest crystal field stabilisation energy?

In potassium ferrocyanide,there are $..........$ pairs of electrons in the $t_{2g}$ set of orbitals.

Number of complexes with an even number of electrons in $t_{2g}$ orbitals is -
$[Fe(H_2O)_6]^{2+}, [Co(H_2O)_6]^{2+}, [Co(H_2O)_6]^{3+}, [Cu(H_2O)_6]^{2+}, [Cr(H_2O)_6]^{2+}$

Zero value of $CFSE$ is observed in which of the following complexes?

In which case is a $d^7$ complex species more stable?