Answer:b. fix nitrogen needed for proteins and nucleic acids.
Explanation: endosymbionts are organisms which lives in the body of other organisms.here the organisms live together and obtain nutrition.An example of this kind of interaction is the formation of root nodules by rhizobium sp.
Nitrogen is abundance in the atmosphere ,but not in the form that plants can readily use.it has to be converted into useful forms .
One of such process of converting it is nitrogen fixation,where atmospheric nitrogen is converted into ammonia and nitrates.the bacteria called rhizobium is involved in this process.it lives in the roots of legumes and can the roots to swell.rhizobium lives in the root nodules, obtaining energy from the plant while fixing nitrogen.