The correct answer is option 'C' - Clostridium.

Clostridium is a genus of anaerobic bacteria that includes several species capable of fixing nitrogen. Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into a usable form, such as ammonia (NH3) or nitrate (NO3-), that can be utilized by plants and other organisms. This process is essential for the growth and development of living organisms, as nitrogen is an essential nutrient.

Here is a detailed explanation of why Clostridium is the correct answer:

1. Anaerobic Bacteria:
- Anaerobic bacteria are microorganisms that can survive and grow in the absence of oxygen.
- Clostridium is a genus of bacteria that are strictly anaerobic, meaning they cannot survive in the presence of oxygen.
- This characteristic allows Clostridium species to thrive in environments where oxygen levels are low, such as deep soil or the root nodules of leguminous plants.

2. Nitrogen Fixation:
- Nitrogen fixation is the process by which atmospheric nitrogen is converted into a form that can be taken up and utilized by living organisms.
- This process is carried out by nitrogen-fixing bacteria, which possess the enzyme nitrogenase that enables them to convert atmospheric nitrogen into ammonia.
- Ammonia can then be used by plants to synthesize amino acids, proteins, and other nitrogen-containing compounds.

3. Clostridium and Nitrogen Fixation:
- Certain species of Clostridium, such as Clostridium pasteurianum and Clostridium acetobutylicum, are known to have the ability to fix nitrogen.
- These bacteria can convert atmospheric nitrogen into ammonia through the action of nitrogenase enzymes.
- Clostridium species that are capable of nitrogen fixation are often found in association with plants, particularly legumes, forming symbiotic relationships.
- For example, Clostridium pasteurianum can form a symbiotic relationship with the legume plant Sesbania rostrata, where the bacteria colonize the plant's root nodules and provide it with fixed nitrogen.

In conclusion, Clostridium is a free-living anaerobic bacterium that is capable of fixing nitrogen. Its ability to convert atmospheric nitrogen into a usable form makes it an important contributor to the nitrogen cycle and an essential player in the ecosystem.

Nitrogen fixing microorganisms are capable of transforming atmospheric nitrogen into fixed nitrogen (inorganic compounds usable by plants) .two kinds of nitrogen fixing bacteria are recognized. the first kind, the free living(non symbiotic) bacteria including cyanobacteria and some genera such as Azotobacter,Clostridium,etc.Clostridium includes the genus of anaerobic, gram positive bacteria. whereas Azotobacter refers to the genus of aerobic gram negative nitrogen fixing bacteria.