Complex species are chemical compounds in which a central metal ion is surrounded by ligands. Optical isomerism occurs when a compound exists in two non-superimposable mirror-image forms, known as enantiomers. In order for a complex species to exhibit optical isomerism, it must possess certain characteristics.

To determine which of the given complex species is not expected to exhibit optical isomerism, we need to analyze the ligands and their arrangement around the central metal ion in each compound.

Let's analyze each option:

a) [Co(en)3]3
- In this compound, there are three ethylenediamine (en) ligands bound to the central cobalt ion. Each ethylenediamine ligand is a bidentate ligand, meaning it can form two bonds with the metal ion. The coordination number of the central cobalt ion is 6. Since the three ethylenediamine ligands are identical and arranged symmetrically around the cobalt ion, this compound is expected to exhibit optical isomerism.

b) [Co(en)2Cl2]
- In this compound, there are two ethylenediamine (en) ligands and two chloride (Cl-) ligands bound to the central cobalt ion. Again, each ethylenediamine ligand is a bidentate ligand. The coordination number of the central cobalt ion is 4. The ligands are arranged in a square planar geometry around the cobalt ion. Since the ligands are not identical and not arranged symmetrically, this compound is expected to exhibit optical isomerism.

c) [Co(NH3)3Cl3]
- In this compound, there are three ammonia (NH3) ligands and three chloride (Cl-) ligands bound to the central cobalt ion. Each ammonia ligand is a monodentate ligand, meaning it can form only one bond with the metal ion. The coordination number of the central cobalt ion is 6. The ligands are arranged in an octahedral geometry around the cobalt ion. Since the ligands are identical and arranged symmetrically, this compound is not expected to exhibit optical isomerism.

d) [Co(en)(NH3)Cl2]
- In this compound, there is one ethylenediamine (en) ligand, one ammonia (NH3) ligand, and two chloride (Cl-) ligands bound to the central cobalt ion. The ethylenediamine ligand is a bidentate ligand, and the ammonia ligand is a monodentate ligand. The coordination number of the central cobalt ion is 5. The ligands are arranged in a trigonal bipyramidal geometry around the cobalt ion. Since the ligands are not identical and not arranged symmetrically, this compound is expected to exhibit optical isomerism.

Therefore, the correct answer is option 'C' [Co(NH3)3Cl3] as it is not expected to exhibit optical isomerism due to the identical and symmetric arrangement of the ligands.