A free-living aerobic and non-photosynthetic nitrogen fixing bacterium is

• A. Anabaena
• B. Clostridium
• C. Azotobacter
• D. Rhizobium

The correct answer is C. Azotobacter. This bacterium is known for its ability to fix nitrogen in the soil while living freely, without the need for photosynthesis.

Key Characteristics of Azotobacter

• Aerobic: Requires oxygen for its metabolic processes.
• Non-photosynthetic: Does not perform photosynthesis to obtain energy.
• Nitrogen-fixing: Converts atmospheric nitrogen into a form usable by plants.

Other Options Explained

To clarify the other options:

• A. Anabaena: A cyanobacterium that is photosynthetic.
• B. Clostridium: An anaerobic bacterium that does not fix nitrogen.
• D. Rhizobium: A nitrogen-fixing bacterium but typically found in symbiotic relationships with legumes.