An octahedral complex of the type $[\mathrm{MX_2A_2}]$, where X are unidentate ligands and A are bidentate ligands (such as ethylenediamine), can exhibit which type(s) of isomerism?

**Geometrical (cis–trans).** The two X ligands can be either adjacent (**cis**) or opposite (**trans**) — that's geometrical isomerism. So option D's 'only linkage' is wrong and option A's 'only optical' is incomplete. **Optical.** The *cis* form is non-superimposable on its mirror image (the two A-A bidentate chelate rings are arranged like the threads of a screw) — so the cis isomer is chiral and exists as a pair of enantiomers. The *trans* form has a mirror plane and is achiral. **Net:** the complex exhibits **both** types — cis–trans plus optical activity within the cis form.