Which of the following is not optically active?

Options

A. (a) `[Co(en)_3]^(3+)`
B. (b) `[Cr(o x)_3]^(3-)`
C. (c) `cis-[CoCl_2(en)_2]^(+)`
D. (d) `trans-[CoCl_2(en)_2]^(+)`

Explanation

To determine which of the given compounds is not optically active, we need to analyze the options based on their molecular geometry and symmetry.

### Step-by-Step Solution:

1. **Understand Optical Activity**:
Optical activity refers to the ability of a compound to rotate plane-polarized light. A compound is optically active if it lacks an internal plane of symmetry and has a chiral center.

2. **Identify the Types of Ligands**:
- The question mentions different ligands (like MAA3 and oxalato) which are bidentate ligands. Bidentate ligands can form chelate complexes with metal ions.

3. **Analyze the Given Options**:
- **Option A (MAA3)**: This compound is symmetrical and exists in only one form, which is optically active.
- **Option B (Oxalato)**: Similar to option A, oxalato is also a symmetrical bidentate ligand and is optically active.
- **Option C and D**: These options represent compounds that can exist in both cis and trans forms.

4. **Cis and Trans Isomerism**:
- In coordination compounds, cis and trans isomers can exhibit different optical activities.
- The **cis form** is typically optically active because it lacks symmetry.
- The **trans form**, however, is usually optically inactive due to its symmetrical arrangement.

5. **Conclusion**:
- Since options A and B are optically active, and option C has both cis (optically active) and trans (optically inactive) forms, the answer to the question "Which of the following is not optically active?" is the **trans form** of the compound in option C or D.

### Final Answer:
The compound that is not optically active is the **trans form** of the coordination compound.