Correct Answer - D
Square planer complexes (1 and 3) never exhibit optical isomerism as the plane in which complex lies is itself a plane of symmetry. Trans -`[Co(en)_(2)C1_(2)]` is optically inative due to the presence of a plane of symmerty. However, sic-`[Co(en)_(2)C1_(2)]^(+)` is optically active as it possesses no symmetry element such as a plane of symmetry.