The correct answer is Azolla pinnata. Nitrogen fixing cyanobacteria (blue green alage) form symbiotic association with several plants,e. g. cycad roots, lichens, liverworts, Azolla. Azolla pinnata is a free floating fresh water form which multiplies rapidly, doubling every 5 - 7 days. Anabaena azolla resides in the leaf cavities of the fern and fixes nitrogen. A part of the fixed nitrogen is excreted in the cavities and becomes available to the fern. The decaying fern plants release the same for utilization of the rice plants. When field is dried at the time of harvesting,the fern functions as the green manure, decomposing and enriching the field for the next crop.