Nitrogen fixation is

Options

A. Converting nitrogen in the air to form a usable form by plants
B. Recycling nitrogen from organic matter in the soil
C. Absorbing nitrogen from the soil.
D. Conversion of `NO_(3)` to `N_(2)`

Answer

### Step-by-Step Text Solution:

1. **Definition of Nitrogen Fixation**: Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into a form that plants can utilize. This process is essential for making nitrogen available to living organisms, as most plants cannot directly use atmospheric nitrogen.

2. **Role of Bacteria**: Certain bacteria, including some species of Rhizobium, play a crucial role in nitrogen fixation. These bacteria can convert free nitrogen in the atmosphere into ammonia (NH₃) or related compounds.

3. **Forms of Nitrogen Absorbed by Plants**: Plants primarily absorb nitrogen in two forms: as nitrate (NO₃⁻) and ammonium (NH₄⁺). These forms are essential for plant growth and development.

4. **Process Overview**: The nitrogen fixation process involves the conversion of atmospheric nitrogen into ammonia, which can then be further transformed into nitrates and ammonium ions that plants can absorb.

5. **Correct Answer Identification**: Among the options provided, the correct answer to the question "What is nitrogen fixation?" is:
- **Option 1**: Converting nitrogen in the air to a form usable by plants.