Correct Answer - D
The overlap of a lone pair on the C atom with the empty hybrid metal orbitals forms a metal-to-carbon `pi`-bond . The transition metal atom in a metal carbonyl has filled non-bonding d-orbitals which are of proper symmetry to overlap with the anti-bonding orbitals of CO . The electronic charge is transferred from the filled non-bonding orbitals of the metal to `pi^(**)` orbitals of the ligand CO . This reduces the bond order of CO . The `pi` back-bonding strengthens the M-C bond order , it weakens the C-O bond order .
