Which of the following does not have optical isomer?

Options

A. (a) `[Co(NH_3)_3Cl_3]`
B. (b) `[Co(en)_3]Cl_3`
C. (c) `[Co(en)_2Cl_2]Cl`
D. (d) `[Co(en)(NH_3)_2Cl_2]Cl`

Explanation

To determine which of the given compounds does not have an optical isomer, we need to analyze the presence of a plane of symmetry in each compound. Optical isomers, or enantiomers, are non-superimposable mirror images of each other, and for a compound to have optical isomers, it must lack a plane of symmetry.

### Step-by-Step Solution:

1. **Understand Optical Isomerism**: Optical isomerism occurs when a compound can exist in two forms that are mirror images of each other. For a compound to exhibit optical isomerism, it must be chiral, meaning it lacks a plane of symmetry.

2. **Identify the Compounds**: Let’s denote the compounds as A, B, C, and D (the specific compounds are not provided in the question, but we will analyze them generically).

3. **Analyze Compound A**:
- Compound A has the following ligands: CO, NH3, Cl, NH3, Cl, NH3.
- If we visualize this compound, we can see that it has a plane of symmetry. For example, if we draw a plane through the center of the molecule, the two halves will be mirror images of each other.
- **Conclusion**: Compound A does not have optical isomers due to the presence of a plane of symmetry.

4. **Analyze Other Compounds (B, C, D)**:
- For the other compounds (B, C, and D), we need to check for the presence of a plane of symmetry.
- If any of these compounds have three bidentate ligands or an arrangement that does not allow for a plane of symmetry, they will be chiral and can exhibit optical isomerism.

5. **Final Conclusion**:
- Based on the analysis, we conclude that **Compound A** is the one that does not have optical isomers because it has a plane of symmetry.

### Answer:
The compound that does not have optical isomer is **Compound A**.