The complex that can show optical activity is :

Options

A. `cis-[CrCl_(2)(o x)_(2)]^(3-)" (ox = oxalate)"`
B. `trans - [Fe(NH_(3))_(2)(CN)_(4)]^(-)`
C. `trans - [Cr(Cl_(2))(o x)_(2)]^(3-)`
D. `cis - [Fe(NH_(3))_(2)(CN)_(4)]^(-)`

Explanation

To determine which complex can show optical activity, we need to analyze the given options based on the presence or absence of a plane of symmetry. Optical activity is a property of a compound that can rotate the plane of polarized light, and it is exhibited by chiral molecules—those that lack a plane of symmetry.

### Step-by-Step Solution:

1. **Understanding Optical Activity**:
- Optical activity refers to the ability of a compound to rotate the plane of polarized light.
- A compound is optically active if it does not possess a plane of symmetry.

2. **Analyzing the Options**:
- We need to examine each complex to see if it has a plane of symmetry.

3. **Option 1: Cis [CrCl2(ox)2]**:
- **Cis Configuration**: In a cis configuration, the two identical ligands (Cl) are adjacent to each other.
- **Structure**: The structure can be visualized as an octahedral complex with Cl ligands on adjacent positions.
- **Plane of Symmetry**: The cis form does not have a plane of symmetry, making it optically active.

4. **Option 2: Trans [CrCl2(ox)2]**:
- **Trans Configuration**: In a trans configuration, the two identical ligands (Cl) are opposite each other.
- **Structure**: This also forms an octahedral complex.
- **Plane of Symmetry**: The trans form has a plane of symmetry, making it optically inactive.

5. **Option 3: [Fe(NH3)4(CN)2]**:
- **Cis Configuration**: Similar to the previous options, this can exist in both cis and trans forms.
- **Plane of Symmetry**: The cis form lacks a plane of symmetry, making it optically active, but the trans form has a plane of symmetry, making it optically inactive.

6. **Option 4: [Fe(NH3)4(CN)2] (Trans)**:
- **Trans Configuration**: This structure will have a plane of symmetry.
- **Plane of Symmetry**: Therefore, it is optically inactive.

7. **Conclusion**:
- Among the options analyzed, **Option 1 (Cis [CrCl2(ox)2])** is the only complex that can show optical activity due to the absence of a plane of symmetry.

### Final Answer:
The complex that can show optical activity is **Option 1**: Cis [CrCl2(ox)2].