The correct answer is b) nitrogen fixation.
Nitrogen fixation is a crucial biological process in which certain bacteria convert atmospheric nitrogen (N2) into ammonia (NH3) or ammonium ions (NH4+). This process allows nitrogen, which is abundant in the atmosphere but not readily usable by most organisms, to be transformed into a form that plants can absorb and utilize for growth.
Several types of bacteria are involved in nitrogen fixation, including free-living bacteria such as Azotobacter and symbiotic bacteria like Rhizobium, which form relationships with legume roots. Without this process, the availability of nitrogen, which is essential for building proteins and nucleic acids, would be significantly limited in ecosystems.