Among the following complexes, the one that can exhibit optical activity is

Options

A. `[CoCl_6]^(3-)`
B. `[Co(e n)Cl_4]^-`
C. cis `-[Co(e n)_2 Cl_2]^(3+)`
D. `trans - [Co(e n)_2Cl_2]^+`

Explanation

To determine which of the given complexes can exhibit optical activity, we need to analyze the structures and ligands of each complex. Optical activity occurs when a compound can rotate plane-polarized light, which typically requires the compound to be chiral (i.e., it lacks an internal plane of symmetry).

### Step-by-Step Solution:

1. **Identify the Complexes**:
- The question mentions several complexes, but we need to focus on their types and ligands to determine chirality.

2. **Understand the Ligands**:
- Ligands can be monodentate (binding through one atom) or bidentate (binding through two atoms). The nature of the ligands affects the symmetry of the complex.

3. **Analyze the MA6 Complex**:
- MA6 complexes typically have octahedral geometry. If all six ligands are the same, the complex will have a plane of symmetry and thus will not be optically active.

4. **Consider MAB Complexes**:
- In MAB complexes, where A is a bidentate ligand and B is a monodentate ligand, the arrangement can lead to chirality if the ligands are arranged in a way that does not allow for a plane of symmetry.

5. **Examine the Specific Complexes**:
- For example, the complex with the formula [Co(en)2Cl2]³⁺ (where "en" is ethylenediamine, a bidentate ligand) can exist in a cis form. The cis form can be chiral if it lacks a plane of symmetry.

6. **Determine Optical Activity**:
- The cis form of [Co(en)2Cl2]³⁺ can exhibit optical activity because it can exist as two non-superimposable mirror images (enantiomers) due to the arrangement of the ligands.

7. **Conclusion**:
- Among the complexes mentioned, the one that can exhibit optical activity is [Co(en)2Cl2]³⁺ in its cis form.