Which of the following compounds shows optical isomerism?

• A

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

  $\left[\mathrm{ZnCl}_4\right]^{-2}$
• Correct answer
  C

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

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

Hence, the correct answer is option: 'C'.