(a) [CoCl2(en)2] shows cis-trans isomerism. (b) [Co(CN)5(NC)]3− is of MA5B type complex, which will not show any isomerism. (c) [Co(NH3)3(NO2)3] shows facial and meridonial isomerism. (d) [Co(NH3)4Cl2]+ shows geometrical (cis-trans) isomerism. In cis isomer, two Cl ligands are adjacent to each other. In trans isomer two Cl ligands are opposite to each other. ∴ Option (b) is correct.