The mosquito fern or Azolla is used as a biofertiliser because it__:

Answer: [C] Has association of nitrogen fixing bluegreen algae

Azolla floats on the surface of water by means of numerous, small, closely overlapping scale-like leaves, with their roots hanging in the water. They form a  symbiotic relationship with the cyanobacterium Anabaena azollae, which fixes atmospheric nitrogen, giving the plant access to the essential nutrient. This has led to the plant being dubbed a “super-plant”, as it can readily colonise areas of freshwater, and grow at great speed – doubling its biomass every two  to three days. The only known limiting factor on its growth is phosphorus, another essential mineral. An abundance of phosphorus, due for example to  eutrophication or chemical runoff, often leads to Azolla blooms. (wikipedia)

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