Which of the following does not show optical isomerism?

Optical isomerism occurs in complexes that lack a plane of symmetry (chiral).The fac- and mer- isomers of $[{\text{MA}}_3{\text{B}}_3]$ type (like $[{\text{Co(NH}}_3{)}_3{\text{Cl}}_3]$) both have planes of symmetry and are therefore achiral and optically inactive.Chelated complexes like $[{\text{Co(en)}}_3{]}^{3+}$ are highly chiral.