During a nuclear fusion reaction,

Options

A. (a) a heavy nucleus breaks into two fragements by itself
B. (b) a light nucleus bombarded by thermal neutrons breaks up
C. (c) a heavy nucleus bombarded by thermal neutrons breaks up
D. (d) two light nuclei combine to give a heavier nucleus and possibly other products

Answer

To solve the question regarding what happens during a nuclear fusion reaction, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Nuclear Fusion**:
- Nuclear fusion is a process where two or more lighter atomic nuclei combine to form a heavier nucleus. This process releases a significant amount of energy.

2. **Analyzing the Options**:
- We have several options to evaluate what happens during a nuclear fusion reaction.

3. **Evaluating Option A**:
- Option A states: "A heavy nucleus breaks into two fragments by itself."
- This describes nuclear fission, not fusion. Therefore, this option is incorrect.

4. **Evaluating Option B**:
- Option B states: "A light nucleus bombarded by thermal neutrons breaks up."
- This also does not describe fusion, as it refers to a reaction involving neutrons and does not involve the merging of nuclei. Thus, this option is incorrect.

5. **Evaluating Option C**:
- Option C states: "A heavy nucleus no, it should be lighter nucleus."
- This is incomplete and does not accurately describe fusion. Hence, this option is also incorrect.

6. **Evaluating Option D**:
- Option D states: "Two light nuclei combined to give a heavier nucleus and possibly other products."
- This accurately describes the process of nuclear fusion, where lighter nuclei combine to form a heavier nucleus. Therefore, this option is correct.

7. **Conclusion**:
- The correct answer is Option D: "Two light nuclei combined to give a heavier nucleus and possibly other products."

### Final Answer:
**Option D** is the correct answer.

---