Cis-trans isomerism is observed in

1. Your syllabus: cis-trans isomerism occurs when two identical ligands can be placed adjacent (cis) or opposite (trans) in the coordination sphere. 2. $[\mathrm{Pt(NH_3)_2\,Cl_2}]$ is SQUARE PLANAR with 2 NH$_3$ and 2 Cl. The two NH$_3$ ligands can either be ADJACENT (cis — same edge) or OPPOSITE (trans — diagonal). Two distinct isomers. 3. cis-platin (the cis isomer) is a famous anticancer drug; the trans isomer is therapeutically inactive — same atoms, different geometry, different biology. 4. Option B has identical ligands all six — no isomerism possible. Option C tetrahedral with 4 identical Cl has no cis/trans choice. Option D is LINEAR — only one arrangement. 5. Note: cis-trans isomerism requires at least TWO different ligand types in a geometry that allows distinct adjacent/opposite placements (square planar with 2:2 or octahedral with 4:2 / 2:2:2 stoichiometries are the typical cases). _Source: syllabus Chemistry Part 1, Ch 5, §5.5.2 (Geometrical isomerism — cis/trans in square planar), p. 8._