Which molecule/ion among the following can't be act as a ligand in complex compounds?
Carbon Monoxide (!!CO!!) forms complex with Nickel.
!!CN^-!! is also a strong ligand. !!Br^-!! also forms complex with metal ions.
Methane (!!CH_4!!) does not act as a ligand as it does not have any lone pair of electrons nor a negative charge.
Therefore, option (d) is correct.