Assertion: When Trytophan is present the repressor is unable to bind to the operator.
Reason: Transcription of structural genes occur.

Options

A. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
B. If both Assertion and Reason are true and the Reason is not the correct explanation of the Assertion.
C. If Assertion is true but the Reason is false.
D. If both Assertion and Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "When tryptophan is present, the repressor is unable to bind to the operator." To analyze this, we need to understand the role of tryptophan in the regulation of gene expression.

**Hint**: Recall the function of tryptophan in the operon model, particularly in the trp operon.

2. **Analyzing the Role of Tryptophan**: In the presence of tryptophan, it actually binds to the repressor protein. This binding changes the shape of the repressor, allowing it to bind to the operator region of the DNA.

**Hint**: Think about how molecules interact in a regulatory mechanism and the effects of binding on protein function.

3. **Understanding the Reason**: The reason states that "Transcription of structural genes occurs." This is typically true when the repressor is not bound to the operator, allowing RNA polymerase to transcribe the structural genes.

**Hint**: Consider what conditions allow for transcription to take place in an operon system.

4. **Connecting Assertion and Reason**: Since the assertion is that the repressor is unable to bind when tryptophan is present, and we have established that tryptophan actually facilitates the binding of the repressor to the operator, the assertion is false. Consequently, if the repressor is bound, transcription of the structural genes will not occur.

**Hint**: Reflect on the relationship between the binding of the repressor and the transcription process.

5. **Final Evaluation**: Both the assertion and the reason are false. The assertion is incorrect because tryptophan presence leads to repressor binding, and the reason is incorrect because if the repressor is bound, transcription does not occur.

**Hint**: Summarize the findings: both statements need to be evaluated based on the mechanisms of gene regulation.

### Conclusion:
Both the assertion and the reason are false. The correct understanding is that when tryptophan is present, the repressor binds to the operator, preventing transcription of the structural genes.