Which of the following coordination compounds shows OPTICAL ISOMERISM?

1. NCERT §5.5.3 (Optical isomerism): a complex shows optical isomerism if it is NON-SUPERIMPOSABLE on its mirror image (chiral). 2. cis-$[\mathrm{Co(en)_2\,Cl_2}]^+$: octahedral, with two bidentate 'en' ligands and two cis-Cl. The geometry is chiral — its mirror image cannot be superimposed by any rotation. So it has $\Delta$ and $\Lambda$ enantiomers. 3. Note that the TRANS isomer of $[\mathrm{Co(en)_2\,Cl_2}]^+$ is achiral (has a plane of symmetry) — so optical isomerism is shown by the CIS isomer only. 4. Option A is square planar with monodentate ligands — has cis/trans isomerism but neither is chiral. 5. Option B has identical ligands all six — symmetric. 6. Option D is tetrahedral with 4 identical CO ligands — symmetric, achiral. _Source: NCERT Class 12 Chemistry Part 1, Ch 5, §5.5.3 (Optical Isomerism in octahedral complexes), p. 8–9._