Which of the following compounds shows optical isomerism?
- $\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{+2}$
- $\left[\mathrm{ZnCl}_4\right]^{-2}$
- $\left[\mathrm{Cr}\left(\mathrm{C}_2 \mathrm{O}_4\right)_3\right]^{-3}$
- $\left[\mathrm{Co}(\mathrm{CN})_6\right]^{-3}$
Asked in: JEE Main 2005