. Free-living nitrogen fixing aerobic bacterium is

Best Answer

Explanation

• The free-living nitrogen-fixing aerobic bacterium is Beijerinckia. It is free-living and can fix nitrogen under aerobic conditions, with the help of the nitrogenase enzyme. 
• The nitrogen-fixing bacteria are mostly prokaryotic which can use atmospheric nitrogen and change it into usable forms like ammonia, nitrates, and nitrites. 
• Rhodospirillum is a facultative anaerobe, thus, depending upon the availability of oxygen, it can either undergo aerobic respiration or alcoholic fermentation. 
• Anabaena is a filamentous bacterium (cyanobacteria), which is free-living or forms a symbiotic association with Azolla (water fern) and has heterocyst cells for nitrogen fixation.
• Nostoc is unicellular cyanobacteria that are found either free-living or live in symbiotic association; for example, coralloid roots of cycas contain symbiotic cyanobacteria Nostoc.
• Rhizobium is a nitrogen-fixing bacteria, commonly found in the root nodules of leguminous plants and fixes nitrogen under anaerobic conditions. 

Final Answer

Hence, the correct option is D i.e., Beijerinckia