Which one of the following is expected to exhibit optical isomerism (en = ethylenediamine)?

• A

  $\mathrm{cis}-\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_2{\mathrm{Cl}}_2\right]$

• B

  $\mathrm{trans}-\left[\mathrm{Co}{\left(\mathrm{en}\right)}_2{\mathrm{Cl}}_2\right]$

• C

  $\mathrm{trans}-\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_2{\mathrm{Cl}}_2\right]$

• D

  $\mathrm{cis}-\left[\mathrm{Co}{\left(\mathrm{en}\right)}_2{\mathrm{Cl}}_2\right]$