The correct answer is Blue green algae. Explanation & Key Takeaways: • Bio-Fertilizers and Their Use in Agriculture:- • For a sustainable agriculture system, it is essential to use renewable inputs (fertilizer, pesticides, water etc.) which can benefit the plant and cause no or minimal damage to the environment. • One of the energy-efficient and pollution-free methods is to exploit the ability of certain microorganisms like bacteria, algae, and fungi to fix atmospheric nitrogen, solubilize phosphorus, decompose organic material or oxidize sulphur in the soil. • When they are applied to the soil, they enhance the growth and yield of crops, improve soil fertility and reduce pollution. They are known as “biofertilizers”. • Blue-green algae:- • Blue-green algae (BGA or cyanobacteria) like Nostoc and Anabaena are free-living photosynthetic organisms also capable of fixing atmospheric nitrogen. • In the flooded rice fields blue-green algae serve as a nitrogen biofertilizer.