1. Identify the geometry of the complexes

All the given cobalt complexes are of coordination number $6$, so they are expected to be octahedral.

2. Condition for optical isomerism in octahedral complexes

A complex shows optical isomerism if it is chiral, i.e. it has no plane of symmetry, center of symmetry, or improper axis.

We now check each option.

3. Option A: $[Co(en)_2Cl_2]^-$

Here, $en$ is ethylenediamine, a bidentate ligand.

For octahedral complexes of type $[M(en)_2a_2]$, two geometrical isomers are possible:

• cis
• trans

Among these:

• the cis form is chiral and shows optical isomerism,
• the trans form is achiral.

So this species can exhibit optical isomerism.

4. Option B: $[Co(NH_3)_3Cl_3]$

This is of type $[MA_3B_3]$ in octahedral geometry.

Such complexes have two geometrical isomers:

• fac
• mer

Both fac and mer forms possess symmetry elements, so they are achiral. Therefore, this complex is not expected to exhibit optical isomerism.

5. Option C: $[Co(en)(NH_3)_2Cl_2]^+$

This is an octahedral complex containing one bidentate ligand ($en$) and two pairs of monodentate ligands.

Such complexes can have arrangements that are chiral (depending on ligand positions), so they may exhibit optical isomerism.

Hence, it is not the required answer.

6. Option D: $[Co(en)_3]^{3+}$

This is the classic octahedral tris-bidentate complex.

Complexes of type $[M(en)_3]^{n+}$ are well known to exist as $\Delta$ and $\Lambda$ optical isomers. So this definitely exhibits optical isomerism.

7. Conclusion

The complex species not expected to exhibit optical isomerism is:

$\boxed{[Co(NH_3)_3Cl_3]}$

So the correct option is B.