According to CFT for ${\mathrm{d}}^6$ configuration in an octahedral complex,  the correct possible electronic configurations  for complex with strong field ligands and with weak  field ligands respectively will be

In octahedral complexes, energy of  ${\mathrm{t}}_{2\mathrm{g}}^{ }<{\mathrm{e}}_{\mathrm{g}} ;$

${\mathrm{\Delta }}_{\mathrm{o}}>\mathrm{P},$strong field ligand, Pairing occurs in lower level until it is completely filled –  ${\mathrm{t}}_{2\mathrm{g}}^6$

${\mathrm{\Delta }}_{\mathrm{o}}<\mathrm{P},$weak field ligand, Pairing occurs only when both ${\mathrm{t}}_{2\mathrm{g}}$ and ${\mathrm{e}}_{\mathrm{g}}$ set of orbitals are half filled…… ${\mathrm{t}}_{2\mathrm{g}}^4{\mathrm{e}}_{\mathrm{g}}^2$