A : Exothermic reactions are possible when two light nuclei fuse or when a heavy nucleus undergoes fission into intermediate mass nuclei.
R: The nature of nuclear binding energy curve is such that it rises lighter nuclei and slightly decreasing for heavier nuclei.

Answer

To analyze the assertion and reason provided in the question, we will break down the concepts of exothermic reactions in nuclear physics and the binding energy curve.

### Step-by-Step Solution:

1. **Understanding Exothermic Reactions**:
- Exothermic reactions are those in which energy is released during the process. In nuclear physics, these reactions can occur in two main scenarios:
- When two light nuclei fuse together (nuclear fusion).
- When a heavy nucleus splits into smaller nuclei (nuclear fission).

2. **Nuclear Fusion**:
- In nuclear fusion, light nuclei (like isotopes of hydrogen) combine to form a heavier nucleus. This process releases a significant amount of energy because the mass of the resulting nucleus is less than the total mass of the original nuclei. The mass difference is converted into energy according to Einstein's equation \(E=mc^2\).

3. **Nuclear Fission**:
- In nuclear fission, a heavy nucleus (like Uranium-235) splits into two or more intermediate mass nuclei. Similar to fusion, this process also releases energy due to the mass defect. The binding energy per nucleon increases as the heavy nucleus breaks apart into more stable intermediate mass nuclei.

4. **Binding Energy Curve**:
- The binding energy curve illustrates how the binding energy per nucleon varies with the mass number of the nucleus.
- For light nuclei, the binding energy per nucleon increases as the nuclei combine (fusion), making them more stable.
- For heavy nuclei, the binding energy per nucleon decreases after reaching a peak (around iron), indicating that they are less stable and can release energy through fission.

5. **Conclusion**:
- The assertion states that exothermic reactions occur during fusion of light nuclei and fission of heavy nuclei, which is correct.
- The reason explains the nature of the binding energy curve, which supports the assertion by showing that lighter nuclei become more stable when fused, and heavier nuclei become unstable when they undergo fission.

### Final Statement:
Both the assertion and the reason are correct, and the reason correctly explains the assertion.

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