Assertion (A): Bacteria come under prokaryotes.
Reason (R): They lack a well-defined nucleus.

Options

A. Both A and R are true, but R is the correct explanation for A.
B. Both A and R are true, but R is the not the correct explanation for A.
C. A is true and R is false.
D. A is false and R is true.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Bacteria come under prokaryotes."
- **Explanation**: Prokaryotes are organisms that do not have a membrane-bound nucleus. Bacteria are classified as prokaryotes because they fit this definition.

2. **Understanding the Reason**: The reason states that "They lack a well-defined nucleus."
- **Explanation**: Bacteria indeed lack a well-defined nucleus. Instead of having a nucleus that is enclosed by a membrane, their genetic material is found in a region called the nucleoid, which is not membrane-bound.

3. **Connecting Assertion and Reason**: Both the assertion and reason are correct statements about bacteria.
- **Explanation**: Since bacteria are classified as prokaryotes due to their lack of a well-defined nucleus, both statements are true.

4. **Evaluating the Explanation**: While both the assertion and reason are correct, the reason does not fully explain why bacteria are classified as prokaryotes.
- **Explanation**: The reason is true, but it does not provide a comprehensive explanation of the classification of bacteria as prokaryotes. Therefore, while both statements are correct, the reason does not adequately explain the assertion.

5. **Conclusion**: Based on the analysis, the correct option would be that both the assertion and reason are correct, but the reason does not correctly explain the assertion.

### Final Answer:
Both the assertion (A) and reason (R) are correct, but R does not explain A.

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