The correct option is D −0.4Δo
The configuration of d6 ocathedral complex at weak field is (t2g)4 (eg)2
CFSE for octahedral complex =[−0.4(n)t2g+0.6(n′)eg ]Δo+n∗P
Where,
n: Number of e− occupying t2g orbital
n': Number of e− occupying eg orbital
Δo: Ligand field splitting energy
n∗: Number of extra electron pairs
P: Spin-pairing energy
CFSE=[−0.4×4+0.6×2]Δo
CFSE=−1.6+1.2=−0.4Δo