A free living, anaerobic nitrogen fixing bacterium in soil is

Options

A. Azotobacter
B. Klebsiella
C. Clostridium
D. Both (2) and (3)

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a free-living, anaerobic nitrogen-fixing bacterium found in soil. This means we need to identify a bacterium that can fix nitrogen without the presence of oxygen.

2. **Identifying Nitrogen-Fixing Bacteria**: Nitrogen-fixing bacteria can be categorized into two types based on their oxygen requirement: aerobic (requiring oxygen) and anaerobic (not requiring oxygen).

3. **Evaluating Options**: We need to evaluate the given options to determine which bacterium is both free-living and anaerobic.

- **Option 1: Acetobacter**: This bacterium is known to be free-living but it fixes nitrogen in the presence of oxygen, making it aerobic. Therefore, it is not the correct answer.

- **Option 2: Klebsiella**: This bacterium is free-living and can fix nitrogen without the need for oxygen, which qualifies it as an anaerobic nitrogen-fixing bacterium.

- **Option 3: Clostridium**: Similar to Klebsiella, Clostridium is also a free-living bacterium that does not require oxygen to fix nitrogen, thus it is also anaerobic.

- **Option 4: Other options**: Since we are looking for a single correct answer, we need to determine which among Klebsiella and Clostridium is the best fit for the question.

4. **Final Conclusion**: Both Klebsiella and Clostridium are correct answers as they both are free-living anaerobic nitrogen-fixing bacteria. However, if the question requires a single answer, we can select either of the two.

5. **Selecting the Correct Answer**: Based on the evaluation, the correct answer to the question is **Klebsiella** or **Clostridium**. If only one answer is required, we can state that **Klebsiella** is a commonly recognized example.

**Final Answer**: Klebsiella (or Clostridium)

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