In Fe(CO)₅, the Fe – C bond possesses

This is known as synergic bonding, in this the ligand CO donates the lone pair of electrons to the iron atom having a vacant orbital and forms a sigma bond. Since the iron atoms also has some electrons in the d-orbitals it will donate those electrons to the molecular orbital of the ligand which leads to the formation of a pie bond. By this process the metal carbon pi bond is reduced and the complex becomes more stable. Hence, the correct answer is option (b).