Explanation:

- Rhizobium:

- Rhizobium bacteria are well-known nitrogen-fixers, primarily found in symbiotic relationships with leguminous plants.

- They form nodules on roots and convert atmospheric nitrogen (N2) into ammonia, which the plant can use.

- E. coli:

- Correct Answer: E. coli does not fix nitrogen.

- Escherichia coli (E. coli) is a common bacterium found in the intestines of humans and animals.

- It is not capable of nitrogen fixation and is primarily known for its role in digestion and as a model organism in research.

- Azotobacter:

- Azotobacter is a free-living nitrogen-fixing bacterium found in the soil.

- It contributes to nitrogen fixation independently, without the need for a symbiotic relationship with plants.

- Cyanobacteria:

- Cyanobacteria, also known as blue-green algae, are capable of nitrogen fixation.

- They perform photosynthesis and are often found in aquatic environments or moist terrestrial conditions.

Correct Answer: E. coli