(A) in alpha particle scattering number of alpha paritcle undergoing head on collision is small.
(R) small fraction of the number of incident particles rebound back.

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.

Explanation

To analyze the statements regarding alpha particle scattering, we need to break down the assertion (A) and the reason (R) step by step.

### Step-by-Step Solution:

1. **Understanding Assertion (A)**:
- The assertion states that "in alpha particle scattering, the number of alpha particles undergoing head-on collision is small."
- This means that when alpha particles are directed towards a target (like a thin foil), only a few of them collide directly with the nucleus of the atoms in the target.

2. **Understanding Reason (R)**:
- The reason states that "a small fraction of the number of incident particles rebound back."
- This means that out of all the alpha particles that hit the target, only a few are deflected back after the collision.

3. **Connecting Assertion and Reason**:
- The reason provided (R) does not directly explain the assertion (A).
- While it is true that a small fraction of particles rebound, the reason for the small number of head-on collisions is due to the fact that the mass of the atom is concentrated in a very small volume. This concentration means that the probability of an alpha particle hitting the nucleus directly (head-on) is low.

4. **Conclusion**:
- Both statements (A) and (R) are true, but the reason (R) does not adequately explain the assertion (A). Therefore, the correct conclusion is that both statements are true, but the reason is not a correct explanation for the assertion.

### Final Answer:
- Both (A) and (R) are true, but (R) is not a correct explanation for (A).