Assertion: Primary transcripts in eukaryotes are nonfunctional.
Reason: Methyl guanosine triphosphate is attached to 5' - end of hnRNA.

Options

A. Both assertion and reason are true, and reason is the correct explanation of assertion.
B. Both assertion and reason are true, but reason is not the correct explanation of assertion.
C. Assertion is true but reason is false.
D. Both assertion and reason are false.

Explanation

To solve the question regarding the assertion and reason, we will analyze both statements step by step.

### Step 1: Analyze the Assertion
**Assertion:** Primary transcripts in eukaryotes are nonfunctional.

- In eukaryotic cells, the primary transcript (also known as heterogeneous nuclear RNA or hnRNA) is synthesized from DNA during transcription.
- This primary transcript contains both exons (coding sequences) and introns (non-coding sequences).
- Because it contains introns, which need to be removed through a process called splicing, the primary transcript is not yet functional as mRNA. Therefore, the assertion is true.

### Step 2: Analyze the Reason
**Reason:** Methyl guanosine triphosphate is attached to the 5' end of hnRNA.

- The primary transcript (hnRNA) undergoes several modifications before it becomes a functional mRNA.
- One of these modifications is the addition of a 5' cap, which is indeed a methyl guanosine triphosphate (m7G) cap.
- This capping is crucial for mRNA stability and for the initiation of translation. Therefore, the reason is also true.

### Step 3: Determine the Relationship Between Assertion and Reason
- The assertion states that primary transcripts are nonfunctional, which is correct because they contain introns that need to be spliced out.
- The reason correctly describes a modification (5' capping) that occurs in hnRNA, but it does not explain why the primary transcript is nonfunctional. The reason is true, but it does not provide a direct explanation for the assertion.

### Conclusion
Based on the analysis:
- Both the assertion and the reason are true.
- However, the reason is not the correct explanation of the assertion.

Thus, the correct answer is: **Assertion and reason are true, but the reason is not the correct explanation of the assertion.**