Among the complexes given below, the compound(s) that exhibit(s) optical isomerism is(are):
(i) cis- $\left[\mathrm{Fe}\left(\mathrm{NH}_3\right)_2(\mathrm{CN})_4\right]^{-}$; (ii) trans- $\left[\mathrm{Fe}\left(\mathrm{NH}_3\right)_2(\mathrm{CN})_4\right]^{-}$; (iii) cis- $\left[\mathrm{CrCl}_2\left(\mathrm{C}_2 \mathrm{O}_4\right)_2\right]^{3-}$; (iv) trans$\left[\mathrm{CrCl}_2\left(\mathrm{C}_2 \mathrm{O}_4\right)_2\right]^{3-}$

• A

  Only (i).
• B

  Both (ii) and (iv).
• Correct answer
  C

  Only (iii).
• D

  Both (i) and (iii)

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