Which one of the following has an optical isomer ?

$\left[\mathrm{Zn}(\mathrm{en})_{2}\right]^{2+}$

$\left[\mathrm{Zn}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}$

$\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}$

$\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{en})\right]^{3+}(\mathrm{en}=$ ethylenediamine)