Answer:
The correct answer would be nitrogen-fixing bacteria.
Nitrogen-fixing bacteria live in symbiotic relationship in the roots of plants especially leguminous plants such as peanuts, soybean, pulses et cetera.
The symbiotic bacteria live in the root nodules of the plants.
It helps in fixing the atmospheric nitrogen and converts into the forms which can be used by plants.
In return, plants provide the bacteria with shelter and food (glucose).
Example of symbiotic bacteria would be rhizobia