Complex species are molecules that consist of a central metal ion surrounded by ligands. Optical isomerism refers to the ability of a molecule to exist in two non-superimposable mirror image forms, known as enantiomers. In order for a complex species to exhibit optical isomerism, it must meet certain criteria.

In this question, we are given four complex species and we need to determine which one is not expected to exhibit optical isomerism. Let's analyze each option.

a) [Co(en)3]3
- This complex species contains three molecules of the ligand ethylenediamine (en) attached to a central cobalt ion. It has an octahedral geometry, and since it contains three identical ligands, it is expected to exhibit optical isomerism.

b) [Co(en)2Cl2]
- This complex species contains two molecules of ethylenediamine and two chloride ions attached to a central cobalt ion. It also has an octahedral geometry, and since it contains two identical ligands, it is expected to exhibit optical isomerism.

c) [Co(NH3)Cl3]
- This complex species contains three molecules of ammonia (NH3) and three chloride ions attached to a central cobalt ion. It has an octahedral geometry, but it does not contain any chiral ligands. In order for a complex species to exhibit optical isomerism, it must have at least one chiral ligand. Since NH3 is not a chiral molecule, [Co(NH3)Cl3] is not expected to exhibit optical isomerism.

d) [Co(en)(NH3)2Cl2]
- This complex species contains one molecule of ethylenediamine, two molecules of ammonia, and two chloride ions attached to a central cobalt ion. It has an octahedral geometry and contains one chiral ligand (en). Therefore, [Co(en)(NH3)2Cl2] is expected to exhibit optical isomerism.

In conclusion, the complex species that is not expected to exhibit optical isomerism is [Co(NH3)Cl3] (option c) because it does not contain any chiral ligands.