Which of the following can exhibit geometrical isomerism (M stands for a metal, and a and b are achiral ligands)?

Geometrical isomerism can be seen in square planar Ma₂b₂ and octahedral Ma₄b₄.

They come in both cis and trans forms. In the complexes ${\mathrm{Ma}}_5$ and ${\mathrm{Ma}}_6$, geometrical isomerism is not possible.

The correct answer is a, b.