Which of the following compounds shows optical isomerism?

• A

  ${\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}$

• B

  ${\left[{\mathrm{ZnCl}}_4\right]}^{2-}$

• C

  ${\left[\mathrm{Cr}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]}^{3-}$

• D

  ${\left[\mathrm{Co}{\left(\mathrm{CN}\right)}_6\right]}^{3-}$