Biofertilizers and microorganisms

Agaricus is a fungus (basidiomycete) used for food production, not as a biofertilizer. Nostoc is a cyanobacterium that fixes atmospheric nitrogen and is used as a biofertilizer, particularly in rice paddies. Spirogyra, while an alga, contains some nitrogen-fixing capabilities and can be used as a biofertilizer. Both Nostoc and Spirogyra are used in agricultural systems to improve soil fertility without chemical fertilizers. Agaricus is excluded. > Key: Cyanobacteria (Nostoc) and certain algae (Spirogyra) are natural nitrogen-fixers used in agriculture. Answer: c.