When a di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it forms a chelate complex. Such complexes, called chelate complexes, tend to be more stable than similar complexes containing only unidentate ligands . This extra stability is known as the chelating effect. Among the given options, only $[CoCl_2(en)_2]NO_3$ contains a chelating ligand, ethylenediamine ($en$), which is a didentate ligand . The other complexes contain only unidentate ligands ($NH_3$, $H_2O$, $Br^-$, $NO_3^-$) . Therefore, $[CoCl_2(en)_2]NO_3$ is the most stable compound.