Identify which of the following is a fusion reaction.

Options

A. `""_1H^2 + ""_1H^2 to ""_2 He^4`
B. `""_1H^2 + ""_1H^2 to 2(""_1 H^2)`
C. `""_1H^1 + ""_1H^1 to ""_2 H^4`
D. `""_1H^1 + ""_1H^2 to ""_2 H^4+n`

Explanation

To identify which of the given reactions is a fusion reaction, we need to understand what a fusion reaction is. A fusion reaction occurs when two lighter atomic nuclei combine to form a heavier nucleus, releasing energy in the process.

### Step-by-Step Solution:

1. **Understanding Fusion Reaction**:
- A fusion reaction involves the combination of two light nuclei to form a heavier nucleus.
- This process typically releases a significant amount of energy, as the mass of the resulting nucleus is less than the sum of the masses of the original nuclei.

2. **Analyzing the Given Reactions**:
- We need to look at each of the four reactions provided and determine if they fit the criteria of a fusion reaction.

3. **Identifying the Correct Reaction**:
- Let's denote the reactions as follows:
- Reaction 1: \( ^2_1H + ^2_1H \rightarrow ^4_2He + \text{energy} \)
- Reaction 2: \( ^1_1H + ^1_1H \rightarrow \text{(unknown)} \)
- Reaction 3: \( ^1_1H + ^2_1H \rightarrow \text{(unknown)} \)
- Reaction 4: \( ^1_1H + ^1_1H \rightarrow \text{(unknown)} \)

4. **Evaluating Each Reaction**:
- **Reaction 1**: This is a classic example of a fusion reaction where two hydrogen nuclei (deuterium) combine to form helium and release energy.
- **Reaction 2**: This reaction does not specify a product, so we cannot confirm if it is a fusion reaction.
- **Reaction 3**: This reaction also does not specify a product, making it impossible to classify.
- **Reaction 4**: Similar to Reaction 2, it does not provide enough information.

5. **Conclusion**:
- The only reaction that clearly demonstrates the characteristics of a fusion reaction is Reaction 1: \( ^2_1H + ^2_1H \rightarrow ^4_2He + \text{energy} \).
- Therefore, the correct answer is **Option 1**.