Which of the following is free-living aerobic non-photosynthetic nitrogen-fixing bacterium?

Explanation

The correct answer is d. Azotobacter. Let's explain each option and provide more details: a. Nostoc: Nostoc is a cyanobacterium, not a bacterium. It is a photosynthetic microorganism capable of fixing atmospheric nitrogen, but it is not a free-living aerobic non-photosynthetic nitrogen-fixing bacterium. b. Azospirillum: Azospirillum is a free-living, nitrogen-fixing bacterium, but it is facultatively anaerobic, meaning it can grow in the absence of oxygen as well. It does not strictly meet the criteria of being aerobic, as specified in the question. c. Rhizobium: Rhizobium is a nitrogen-fixing bacterium, but it forms a symbiotic relationship with leguminous plants and is not free-living. It establishes nodules on the roots of legumes and fixes nitrogen in this symbiotic association. d. Azotobacter: Azotobacter is a free-living, aerobic, and non-photosynthetic nitrogen-fixing bacterium. It can be found in various soil and aquatic environments and plays a crucial role in nitrogen cycling by converting atmospheric nitrogen into a form that plants can use for growth. Summary: Among the options provided, Azotobacter is the free-living aerobic non-photosynthetic nitrogen-fixing bacterium, and it is known for its ability to fix atmospheric nitrogen independently in various environments.