A nitrogen fixing bacterium that forms a loose assocation with the roots of crop plants is
Or
Which one of the following micro-organisms is used as a bio fertilizer?
(a) Azotobacter
(b) Bacillus polymyxa
(c) Clostridium
(d) Azospirillum

Options

A. Azotobacter
B. Bacillus polymyxa
C. Clostridium
D. Azospirillum

Answer

To solve the question regarding the nitrogen-fixing bacterium that forms a loose association with the roots of crop plants and is used as a biofertilizer, we will analyze each of the given options:

1. **Understanding the Question**:
- We need to identify a nitrogen-fixing bacterium that forms a loose association with crop plant roots and is used as a biofertilizer.

2. **Analyzing the Options**:
- **(a) Azotobacter**: This bacterium is known for nitrogen fixation but does not form a significant association with plant roots. It is primarily free-living in the soil.
- **(b) Bacillus polymyxa**: This bacterium is beneficial for soil health but does not specifically fix nitrogen or form a loose association with plant roots.
- **(c) Clostridium**: While this is a nitrogen-fixing bacterium, it is mostly anaerobic and does not form associations with plant roots.
- **(d) Azospirillum**: This bacterium is known for forming a loose association with the roots of various crop plants and is widely used as a biofertilizer due to its ability to fix nitrogen and enhance soil fertility.

3. **Conclusion**:
- Based on the analysis, the correct answer is **(d) Azospirillum**. It is the only option that meets both criteria of forming a loose association with crop plant roots and being used as a biofertilizer.

### Final Answer:
**(d) Azospirillum**