Which of the following has an optical isomer? (en = ethylene diamine)?

• A

  ${\left[\mathrm{Zn}\left(\mathrm{en}\right){\left({\mathrm{NH}}_3\right)}_2\right]}^{2+}$

• B

  ${\left[\mathrm{Co}{\left(\mathrm{en}\right)}_3\right]}^{3+}$

• C

  ${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4\left(\mathrm{en}\right)\right]}^{3+}$

• D

  ${\left[\mathrm{Zn}{\left(\mathrm{en}\right)}_2\right]}^{2+}$