The correct answer is:
2 only - Nostoc.
Nostoc is a cyanobacterium (blue-green algae) that is used as a biofertilizer. It has the ability to fix atmospheric nitrogen and convert it into a form that plants can utilize, thereby enhancing soil fertility.
Agaricus and Spirogyra are not typically used as biofertilizers. Agaricus is a genus of mushrooms, and Spirogyra is a filamentous green algae. While both organisms have ecological importance, they are not commonly employed as biofertilizers.
Therefore, the correct answer is "2 only - Nostoc."