a
There is unsymmetric filling of \(e_{g}\) subset of \(Cu ^{+2} \;ion\), while there is symmetrical distribution in \(t _{2 g }\) set, if the complex has same ligand there will be equal repulsion which leads to symmetrical bond length along \(t _{2 g }\) , but due to uneven filling of electron in \(e _{ g }\) subset, either octahedral will be elongated or compressed.