Which of the following exercise a control over transcription :

Options

A. Operator
B. Regulator
C. Promoter
D. Recon

Answer

To answer the question "Which of the following exercises a control over transcription?" we will analyze each of the options provided: operator, regulator, promoter, and recon.

### Step-by-Step Solution:

1. **Understanding Transcription**:
- Transcription is the process by which RNA is synthesized from a DNA template. Specifically, it involves the formation of messenger RNA (mRNA) from DNA.

2. **Analyzing the Options**:
- **Operator**:
- The operator is a segment of DNA that a repressor protein binds to. When the repressor is bound to the operator, it prevents transcription from occurring. Therefore, while the operator is involved in the regulation of transcription, it does not directly control the transcription process itself.

- **Regulator**:
- The regulator refers to a gene that produces a protein (often a repressor or activator) which can control the activity of the operator. It plays a significant role in regulating whether transcription is turned on or off. Thus, the regulator does exercise control over transcription.

- **Promoter**:
- The promoter is the region of DNA where RNA polymerase binds to initiate transcription. While it is crucial for the start of transcription, it does not control the process in the sense of regulating whether transcription occurs or not.

- **Recon**:
- Recon is defined as the smallest unit of recombination and is not directly related to the transcription process. It does not play a role in controlling transcription.

3. **Conclusion**:
- Based on the analysis, the correct answer is **Regulator**, as it is the component that exercises control over the transcription process by regulating the operator's activity.

### Final Answer:
**Regulator**

---