Ligand in a metal carbonyl complex

$(a)\;CO_2\qquad(b)\;CO\qquad(c)\;COCl_2\qquad(d)\;C_2O_4^{2-}$

Ligand in a metal carbonyl complex is CO
Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands
Hence (b) is the correct answer.
edited Nov 8, 2017