A free-living bacterium capable of fixing nitrogen is:
(a) Rhizobium
(b) Streptococcus
(c) Pseudomonas
(d) Azotobacter

Options

A. Rhizobium
B. Streptococcus
C. Pseudomonas
D. Azotobacter

Explanation

To solve the question of identifying a free-living bacterium capable of fixing nitrogen, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for a bacterium that is both free-living and capable of fixing nitrogen.

2. **Identify the Options**: The options given are:
- (a) Rhizobium
- (b) Streptococcus
- (c) Pseudomonas
- (d) Azotobacter

3. **Evaluate Each Option**:
- **Rhizobium**: This bacterium is known for its role in nitrogen fixation, but it is not free-living; it forms symbiotic relationships with leguminous plants, creating root nodules. Therefore, it does not fit the criteria.
- **Streptococcus**: This is a genus of bacteria that does not fix nitrogen. It is primarily known for its role in human diseases and does not meet the criteria.
- **Pseudomonas**: While some species of Pseudomonas can be involved in nitrogen cycling, they are not primarily known for nitrogen fixation and are not free-living nitrogen-fixers.
- **Azotobacter**: This is a free-living bacterium that is well-known for its ability to fix nitrogen from the atmosphere into a form that plants can use. It meets both criteria of being free-living and capable of nitrogen fixation.

4. **Conclusion**: Based on the evaluation, the correct answer is:
- (d) Azotobacter

### Final Answer:
(d) Azotobacter