The process that is the opposite of nitrogen fixation is

Options

A. (a) Nitrification
B. (b) Denitrification
C. (c) Ammonification
D. (d) Nitrate reduction

Explanation

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds in soil, making it available for plants. This process is crucial for the nitrogen cycle and is primarily carried out by certain bacteria.

2. **Identifying the Opposite Process**:
- To find the process that is opposite to nitrogen fixation, we need to consider what happens to nitrogen after it has been fixed into forms that plants can use.

3. **Exploring Biological Processes**:
- The options provided are:
- A) Nitrification
- B) Denitrification
- C) Ammonification
- D) Nitrate Reduction
- Among these, we need to identify which process involves the conversion of nitrogen compounds back into atmospheric nitrogen.

4. **Defining Denitrification**:
- Denitrification is the process where nitrates (NO₃⁻) and nitrites (NO₂⁻) in the soil are converted back into nitrogen gas (N₂) by denitrifying bacteria. This process effectively removes nitrogen from the soil and returns it to the atmosphere.

5. **Conclusion**:
- Since nitrogen fixation involves the conversion of atmospheric nitrogen into forms usable by plants, and denitrification is the process that converts these forms back into atmospheric nitrogen, denitrification is indeed the opposite of nitrogen fixation.

6. **Final Answer**:
- Therefore, the correct answer to the question "The process that is the opposite of nitrogen fixation is" is **Option B: Denitrification**.