`N_(2)` fixation is

Options

A. (a) `N_(2) to NH_(3)`
B. (b) `NO_(3) to NO_(2)`
C. (c) `N_(2) to `amino acid
D. (d) both (a) and (b)

Answer

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into a more usable form for living organisms, primarily ammonia (NH₃).

2. **First Reaction**:
- The first step in nitrogen fixation involves the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃). This is typically carried out by certain bacteria and archaea, which possess the enzyme nitrogenase.

3. **Intermediate Form**:
- Ammonia (NH₃) serves as an intermediate organic form of nitrogen. It can be utilized by plants to synthesize nitrogenous compounds.

4. **Nitrification Process**:
- After ammonia is formed, it can undergo nitrification, which is a two-step process:
- First, ammonia is converted to nitrite (NO₂⁻).
- Then, nitrite is further converted to nitrate (NO₃⁻), which is the most common and usable form of nitrogen for plants.

5. **Evaluating the Options**:
- **Option A**: Conversion of inorganic nitrogen to ammonia. This is correct as it describes the initial step of nitrogen fixation.
- **Option B**: Conversion of nitrate to nitrite. This is incorrect because it describes a process that occurs during nitrification, not nitrogen fixation.
- **Option C**: Conversion of inorganic nitrogen to amino acids. This is also incorrect as there’s no direct conversion; it first goes through ammonia and nitrite.
- **Option D**: Both A and B. Since B is incorrect, this option is also incorrect.

6. **Conclusion**:
- The correct answer is **Option A**: Conversion of inorganic nitrogen to ammonia.