The five d-orbitals are classified based on their orientation relative to the coordinate axes:

1. Axial Orbitals ($e_g$ set): The $d_{x^2-y^2}$ and $d_{z^2}$ orbitals have their electron density lobes directed along the axes. The $d_{x^2-y^2}$ orbital lies along the x and y axes, while the $d_{z^2}$ orbital lies along the z-axis.
2. Non-Axial Orbitals ($t_{2g}$ set): The $d_{xy}$, $d_{yz}$, and $d_{xz}$ orbitals have their lobes oriented between the axes (at 45 degrees to the axes).

Therefore, the pair with electron density along the axes is $d_{z^2}$ and $d_{x^2-y^2}$ .