[A] : SA node is contraction centre or Pace Maker.
[R]: Nodal fibres of SA node are embeded in the wall of right atrium

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that the SA node is the contraction center or pacemaker of the heart. The SA node, or sinoatrial node, is indeed responsible for initiating electrical impulses that lead to heart contractions. This makes the assertion true.

2. **Understanding the Reason**: The reason provided states that the nodal fibers of the SA node are embedded in the wall of the right atrium. This is also true, as the SA node is located in the upper right corner of the right atrium, where it generates impulses.

3. **Evaluating the Relationship**: Although both the assertion and the reason are true, we need to determine if the reason correctly explains the assertion. The assertion focuses on the function of the SA node as the pacemaker, while the reason describes its anatomical location. The anatomical location does not explain why the SA node is the pacemaker.

4. **Conclusion**: Since both the assertion and reason are true, but the reason does not serve as the correct explanation for the assertion, we conclude that the correct answer is that both are true, but the reason is not the correct explanation of the assertion.

### Final Answer:
Both assertion and reason are true, but the reason is not the correct explanation of the assertion.