Tetrahedral complexes of the type $[\mathrm{Ma_2b_2}]$ (where a and b are unidentate ligands) do **not** exhibit geometrical isomerism because:

**Geometrical isomerism** requires distinguishable spatial positions — e.g. in an octahedron, two ligands can be either next-to-each-other (cis, 90°) or opposite (trans, 180°). **In a tetrahedron**, every pair of vertices is equivalent — all four corners are mutually 109.5° apart, so there is no analogous "cis vs trans" distinction. Two pairs of identical ligands can be arranged only one way (up to rotation). - Option B is wrong: certain tetrahedral $[\mathrm{M(AB)_2}]$ complexes actually *are* optically active. - Option D is plainly false — tetrahedral complexes like $[\mathrm{NiCl_4}]^{2-}$ and $[\mathrm{CoCl_4}]^{2-}$ are very common. **Square planar exception.** $[\mathrm{Ma_2b_2}]$ in *square planar* geometry **does** show cis–trans isomerism (e.g. cisplatin vs transplatin). The difference is the 90° vs 180° distinction available on a square but not on a tetrahedron.