Assertion: Rheumatoid arthritis is an auto- immune disease.
Reason: Body attack self-cells in Rheumatoid arthritis.

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Rheumatoid arthritis is an autoimmune disease."
- **Explanation**: Rheumatoid arthritis (RA) is indeed classified as an autoimmune disease. In autoimmune diseases, the immune system mistakenly attacks the body's own cells.

2. **Understanding the Reason**: The reason provided is "Body attacks self-cells in Rheumatoid arthritis."
- **Explanation**: In rheumatoid arthritis, the immune system produces antibodies that target the body's own joint cells, leading to inflammation and damage in the joints.

3. **Evaluating the Truth of the Assertion**: Since rheumatoid arthritis is characterized by the immune system attacking the joints, the assertion is true.

4. **Evaluating the Truth of the Reason**: The reason correctly describes the mechanism of rheumatoid arthritis, where the body’s immune system attacks its own cells. Thus, the reason is also true.

5. **Determining the Relationship**: The reason provides a correct explanation for the assertion. The immune response in rheumatoid arthritis is indeed a direct attack on self-cells, which aligns with the definition of an autoimmune disease.

6. **Conclusion**: Both the assertion and the reason are true, and the reason is a correct explanation of the assertion. Therefore, the correct option is that both the assertion and reason are true, and the reason is the correct explanation.

### Final Answer:
Both the assertion and reason are true, and the reason is the correct explanation of the assertion.

---