Which of the following is a non-symbiotic nitrogen fixing bacteria?
A) Rhizobium
B) Azospirillum
C) Azotobacter
D) Nitrosomonas

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Hint: Prokaryotic organisms which have the capability to convert naturally present nitrogen gas into fixed nitrogen so that plants can use are known as nitrogen fixing bacteria.

Complete Answer:
Let us first understand the concept of nitrogen fixation in plants. Plants cannot use the nitrogen that is available naturally in the atmosphere. The process by which the molecular nitrogen is converted to ammonia or any other nitrogenous compounds is known as nitrogen fixation. Nitrogen fixation can either happen naturally or with the help of some microbes.

Option a: Rhizobium lives in the roots of legumes and thus is an example of symbiotic nitrogen fixing bacteria. Therefore, this is the incorrect option.

Option b: Azospirillum lives in the roots of crops and thus is an example of symbiotic nitrogen fixing bacteria. Therefore, this is the incorrect option.

Option c: Azotobacter is a freelivig bacteria which is present in the soil. They are aerobic and can survive only if there is presence of oxygen. It helps in fixing nitrogen by converting atmospheric nitrogen to ammonia. Thus, it is an example of non-symbiotic nitrogen fixing bacteria. Therefore, this is the correct option.

Option d: Nitrosomonas live in close association to the plants and thus is an example of symbiotic nitrogen fixing bacteria. Therefore, this is the incorrect option.

Thus, the correct option is c) Azotobacter.

Note: Nitrogen fixing bacteria are of two types- free living or non-symbiotic and symbiotic bacteria.
Free living bacteria live in soil whereas symbiotic bacteria live in the root nodules of the plant.