Assertion . True nucleus is absent in E. coli and other prokaryotes.
Reason. An undifferentiated , unorganised, fibrillar nucleus without any limiting membrane is found in prokaryotic cells.

Options

A. both are true with reason being correct explanation
B. both true but reason is not correct explanation
C. assertion true but reason is wrong
D. both are wrong.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that a true nucleus is absent in E. coli and other prokaryotes.
- In prokaryotic cells, such as E. coli, the genetic material is not enclosed within a membrane-bound nucleus, which is characteristic of eukaryotic cells.

2. **Understanding the Reason**:
- The reason provided states that there is an undifferentiated, unorganized, fibrillar nucleus without any limiting membrane found in prokaryotic cells.
- This refers to the nucleoid, which is the region in prokaryotic cells where the genetic material (DNA) is located. Unlike a true nucleus, the nucleoid is not surrounded by a nuclear membrane.

3. **Evaluating the Truth of the Statements**:
- The assertion is true because prokaryotes, including E. coli, do not have a true nucleus.
- The reason is also true as it accurately describes the nucleoid, which is indeed an unorganized mass of DNA without a membrane.

4. **Determining the Relationship**:
- The reason provides a correct explanation for the assertion. The absence of a true nucleus in prokaryotes is explained by the presence of the nucleoid, which is a different structure.

5. **Conclusion**:
- Both the assertion and the reason are true, and the reason correctly explains the assertion.

### Final Answer:
- **Assertion**: True
- **Reason**: True
- **Relationship**: The reason is a correct explanation of the assertion.