Which of the following coordination compounds is incapable of showing geometrical isomerism?

[Co(en)₃]³⁺

Geometrical isomerism is common in square planar and octahedral complexes. 

Square planar complexes with general formula [MA₂X₂]^±n, [MA₂XY]^±n, [MABX₂]^±n, [MABXY]^±n, [M(AB)₂]^±n show isomerism

Octahedral complexes with general formula [MA₄X₂], [MA₄XY], [M(AA)₂X₂] shows geometrical isomerism.