To determine which octahedral complex of the formula \( MA_2B_2C_2 \) can exhibit optical isomerism, we need to analyze the possible combinations of ligands and their spatial arrangements. Optical isomerism occurs when a molecule can exist in two non-superimposable mirror images, known as enantiomers.
### Step-by-Step Solution:
1. **Understanding the Formula**: The formula \( MA_2B_2C_2 \) indicates that the metal \( M \) is coordinated to two types of ligands \( A \), \( B \), and \( C \). Each type of ligand appears twice.
2. **Identifying Ligands**: We need to evaluate the combinations of ligands given in the options to see if they can create a chiral environment.
3. **Analyzing Each Option**:
- **Option A**: \( A = Cl_2 \), \( B = en \), \( C = en \)
- Here, both \( B \) and \( C \) are identical bidentate ligands (ethylenediamine). The arrangement will lead to a symmetrical structure, which cannot produce optical isomers.
- **Option B**: \( A = EDTA \), \( B = EDTA \), \( C = EDTA \)
- All ligands are identical (EDTA), leading to a symmetrical structure. No optical isomerism is possible.
- **Option C**: \( A = oxalato \), \( B = oxalato \), \( C = en \)
- Both \( A \) and \( B \) are bidentate ligands (oxalato), and \( C \) is also a bidentate ligand (en). The arrangement will not create a chiral center, as the two oxalato ligands will create a symmetrical structure.
- **Option D**: \( A = H_2O \), \( B = NH_3 \), \( C = Cl \)
- In this case, we have two different types of monodentate ligands (water and ammonia) and one type of halide ligand (chloride). If we arrange the ligands in a cis configuration, we can create a chiral environment. The mirror image of this arrangement will not be superimposable on the original, thus allowing for optical isomerism.
4. **Conclusion**: The only combination that allows for optical isomerism is **Option D**, which consists of two water molecules, two ammonia molecules, and two chloride ions.
### Final Answer:
The correct option that can exhibit optical isomerism is **Option D**.
