For nitrogen fixation in soil, we may use ______?
A. Cyanobacteria
B. Protozoans
C. Nematodes
D. Wheat plants

Hint: Certain microorganisms play a major role in fixing atmospheric nitrogen. A photosynthetic bacterium that is bluish in colour can convert the nitrogen in the atmosphere into usable organic form with the help of some special cells called heterocysts. This bacterium is predominantly found in water and moist soils.

Complete answer:
Nitrogen is present in the atmosphere in a molecular form, which cannot be used up by plants. So, with the help of nitrogen fixation, this form of the nitrogen gets converted into ammonia and nitrate. It is easier for plants and other organisms to metabolise ammonia than its molecular form.

Cyanobacteria are photosynthetic bacteria that can be found in water or moist soils. They are bluish-green in colour due to the presence of phycocyanin (blue pigment) and chlorophyll-a (green pigment). Both these pigments help in capturing the light energy from the sun.

Nitrogen fixation cannot be done in the presence of oxygen, hence anaerobic conditions are required. This is achieved with the help of heterocysts. Heterocysts are special nitrogen-fixing cells that are formed during nitrogen starvation. These cells fix nitrogen with the help of an enzyme called nitrogenase.

Hence, the correct answer is option (A).

Note: The enzyme nitrogenase is only activated in anaerobic conditions. The fixed nitrogen is generally in the form of ammonia, nitrates, and nitrites which are easily absorbed by plants and converted into proteins and amino acids. Examples of cyanobacteria are Nostoc, Anabaena, Oscillatoria, Spirulina, and Lyngbya. Spirulina is high in protein content and is majorly used as a food source. On the other hand, Lyngbyas are problematic bacteria as they can cause skin irritations.