The following represents a pair of enantiomers:

Options

A. a.trans-`[CrCl_(2)(en)_(2)]^(o+)`
B. b.`cis-[CrCl_(2)(en)_(2)]^(o+)`
C. c.trans-`[CrCl_(2)(NH_(3))_(4)]^(o+)`
D. d.`cis-[CrCl_(2)(NH_(3))_(4)]^(o+)`

Explanation

To determine which of the given options represents a pair of enantiomers, we need to understand the concepts of chirality, enantiomers, and the role of ligands in coordination compounds.

### Step-by-Step Solution:

1. **Understanding Enantiomers**:
- Enantiomers are a pair of molecules that are non-superimposable mirror images of each other. This means that if you were to place one molecule over the other, they would not align perfectly.

2. **Identifying Chiral Centers**:
- A chiral center (or chiral carbon) is typically a carbon atom that is bonded to four different groups. In coordination compounds, the presence of certain ligands can create chirality.

3. **Analyzing the Given Ligand**:
- The ligand mentioned is ethylene diamine (en), which is a bidentate ligand. This means it has two donor atoms (in this case, two nitrogen atoms) that can coordinate to a metal center.

4. **Considering the Coordination Compound**:
- The compound in question is likely a chromium complex with the formula CrCl2(en)2. The arrangement of the ligands around the chromium center can lead to different isomers.

5. **Cis and Trans Isomerism**:
- In coordination complexes, the arrangement of ligands can lead to cis and trans isomers.
- The cis form has similar ligands on the same side, while the trans form has them on opposite sides.

6. **Determining Optical Activity**:
- The cis form of the complex CrCl2(en)2 can exhibit chirality because it can form a non-superimposable mirror image, making it optically active.
- Conversely, the trans form does not exhibit chirality because it has a superimposable mirror image, meaning it cannot be optically active.

7. **Conclusion**:
- Therefore, the correct answer is that the cis form of CrCl2(en)2 represents a pair of enantiomers, while the trans form does not.

### Final Answer:
The following represents a pair of enantiomers: **Cis form of CrCl2(en)2**.

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