(N/A) In an octahedral crystal field,the five degenerate $d$ orbitals of the metal ion split into two sets due to the approach of ligands along the axes.
$1$. The $d_{x^{2}-y^{2}}$ and $d_{z^{2}}$ orbitals,which point directly towards the ligands,experience greater electrostatic repulsion and thus have higher energy,forming the $e_{g}$ set.
$2$. The $d_{xy}$,$d_{yz}$,and $d_{zx}$ orbitals,which point between the axes,experience less repulsion and thus have lower energy,forming the $t_{2g}$ set.
$3$. The energy difference between these two sets is denoted by $\Delta_{o}$ (crystal field splitting energy in octahedral field).