The right answer is B.
Nitrogen fixation is done by decomposers that are prokaryotes (bacteria).
<u>Take the example of legumes.
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The different species of rhizobia bacteria, which are able to infect the roots of legumes (Fabaceae family), are specific to certain host plants. Rhizobium phaseoli infects beans (Phaseolus sp).
Bacteria cause the formation of nodules on the roots by penetrating the root hairs, and transform into larger "bacteroids". The nodules are the seat of a symbiotic activity in which the plant provides the sugars and energy from photosynthesis, and in return benefits from the amino acids that are produced there.
