Assertion: For the scattering of `alpha`-particles at a large angles, only the nucleus of the atom is responsible.
Reason: Nucleus is very heavy in comparison to electrons.

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 assertion and reason provided in the question, we will break down the concepts involved step by step.

### Step 1: Understanding the Assertion
The assertion states that for the scattering of alpha particles at large angles, only the nucleus of the atom is responsible.

**Explanation**:
- Alpha particles are positively charged particles, and when they approach an atom, they interact with the positively charged nucleus.
- The nucleus is much more massive than the electrons surrounding it, which means that when an alpha particle comes close to an atom, the interaction is predominantly with the nucleus rather than the electrons.

### Step 2: Understanding the Reason
The reason given is that the nucleus is very heavy in comparison to electrons.

**Explanation**:
- The mass of the nucleus is significantly greater than that of electrons.
- This mass difference means that the nucleus can exert a much stronger influence on the trajectory of the alpha particles than the electrons can.
- The electrons are located at a distance from the nucleus and are much lighter, so their effect on the scattering of alpha particles is negligible compared to that of the nucleus.

### Step 3: Analyzing the Relationship Between Assertion and Reason
Now we need to determine if the reason provided supports the assertion.

**Explanation**:
- Since the nucleus is indeed very heavy compared to electrons, it is responsible for the scattering of alpha particles at large angles.
- The heavy nucleus can effectively repel the incoming alpha particles due to the electrostatic force, leading to significant deflections (large angle scattering).
- Therefore, the reason correctly explains why the assertion is true.

### Conclusion
Both the assertion and the reason are true, and the reason is a correct explanation of the assertion.

### Final Answer
**Both the assertion and reason are true, and the reason is the correct explanation for the assertion.**

---