Which of the following complex species is not expected to exhibit optical isomerism?

Optical isomerism occurs in complexes that are chiral (lack a plane of symmetry).$[{\text{Co(en)}}_3{]}^{3+}$ is a chelating complex and is always chiral.$[{\text{Co(en)}}_2{\text{Cl}}_2{]}^{+}$ (cis-form) is chiral.$[{\text{Co(NH}}_3{)}_3{\text{Cl}}_3]$ is of the type ${\text{MA}}_3{\text{B}}_3$. It has two geometrical isomers: facial (fac) and meridonial (mer). Both fac and mer isomers possess a plane of symmetry and are therefore achiral. They do not show optical isomerism.