Explanation:

Cyanobacteria

• Cyanobacteria are the only prokaryotes capable of using sunlight as their energy, water as an electron donor, and air as a source of carbon and, for some nitrogen- fixing strains, nitrogen. They live in the water, and can manufacture their own food. Because they are bacteria, they are quite small and usually unicellular, though they often grow in colonies large enough to see. They have the distinction of being theoldest known fossils.
• Because they are photosynthetic and  aquatic, cyanobacteria are often called “blue-green algae”.
• All the statements are correct: A variety of cyanobacteria are mainly responsible for nitrogen fixation in flooded rice fields and bean cultivation in the tropics.
• Few examples of Cyanobacteria are: Gloeocapsa, Oscillatoria, Anabaena, Aulosira, Lyngbya and Nastoc rank amongst the important nitrogen-fixation cyanobacteria.