Which of the following is an example of a biofertilizer that enriches soil nitrogen content?
Nostoc, a nitrogen-fixing cyanobacterium, improves soil fertility by fixing atmospheric nitrogen.
Ref: NCERT Class 12 Biology Chapter 8: Microbes in Human Welfare Microbes as Biofertilizers - Rhizobium Azospirillum Mycorrhiza Cyanobacteria