A free living nitrogen fixing cyanobacterium which can also form symbiotic association with the water fern Azolla is

Explanation

a)Tolypothrix: Tolypothrix is another genus of filamentous cyanobacteria, but it is not typically associated with the water fern Azolla. It does not have the same symbiotic relationship as Anabaena. b)Chlorella: Chlorella is a green alga, not a cyanobacterium. While it can form symbiotic associations with certain organisms, it is not known for its nitrogen-fixing abilities. c)Nostoc: Nostoc is a genus of cyanobacteria, and like Anabaena, it is capable of nitrogen fixation. However, it is not commonly associated with the water fern Azolla. d)The free-living nitrogen-fixing cyanobacterium that can also form a symbiotic association with the water fern Azolla is "Anabaena." Anabaena is known for its ability to fix atmospheric nitrogen and establish symbiotic relationships with certain plants, including Azolla. This symbiotic association can benefit both the cyanobacterium and the host plant through nitrogen fixation and improved nitrogen nutrition. In summary, the correct answer is "Anabaena," as it is a well-known nitrogen-fixing cyanobacterium that can form a symbiotic relationship with Azolla. The other options are not typically associated with this specific symbiosis.