Select the correct statement.

• A

  Geometrical isomer may differ in dipole moment and visible/UV spectra.
• B

  Complexes of the type $\left[\mathrm{Ma}_3 b_3\right]$ can also have facial $(\mathrm{fac})$ and meridional (mer) isomer.
• C

  No optical isomer exists for the complex trans- $\left[\mathrm{Co}(e n)_2 \mathrm{Cl}_2\right]^{+}$.
• Correct answer
  D

  All of these.

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