Biofertilizers and microorganisms

Agaricus is a fungus used for food production (mushrooms), not as a biofertilizer. Nostoc is a cyanobacterium (blue-green alga) that fixes atmospheric nitrogen and is used as a biofertilizer in agriculture, particularly in rice paddies. Spirogyra is an algae that, while photosynthetic, is not typically used as a biofertilizer in agricultural practice. Only Nostoc among these organisms is recognized as an effective biofertilizer due to its nitrogen-fixing capability. > Nostoc = nitrogen-fixing cyanobacterium = biofertilizer. Agaricus = fungus = not a biofertilizer. Spirogyra = algae = not commonly used as biofertilizer. Answer: (b).