Option (C) is correct.
Synergic bonding involves transference of electrons form ligands to metal and The transference of electrons from filled metal orbitals to anti-bonding orbitals of ligands.
its a bond between a carbonyl group acting as a ligand and a metal.Synergic bonding means self strengthening bond.
It is formed by pi-acid ligands like CO.pi-acid ligands are those which have a lone pair of electrons to donate to metal atom and an empty antibonding molecular orbital to back bond with the d-orbital electrons of metal atom
