Cis-trans isomerism is found in square planar complexes of molecular formula: (a and b are monodentate ligands)

• A

  ${\mathrm{Ma}}_4$

• B

  ${\mathrm{Ma}}_3\mathrm{b}$

• C

  ${\mathrm{Ma}}_2{\mathrm{b}}_2$

• D

  ${\mathrm{Mab}}_3$