Nostoc, a genus of cyanobacteria, is utilized as a biofertilizer due to its ability to fix atmospheric nitrogen, which is essential for plant growth. It forms symbiotic relationships with various plants, enhancing soil fertility and promoting sustainable agricultural practices. In contrast, Agaricus refers to a genus of fungi (mushrooms), and Spirogyra is a green alga; neither of these has significant roles in nitrogen fixation or soil fertility improvement, which is why only Nostoc is considered a biofertilizer among the given organisms.