Final answer:
The correct answer to the question is nitrogen fixation (b), which is the process whereby nitrogen-fixing bacteria convert unreactive atmospheric nitrogen gas (N2) into a form that plants can use, such as ammonia (NH3).
Step-by-step explanation:
The process that involves the fixing of nitrogen into a usable form for plants is known as nitrogen fixation. This process is crucial because atmospheric nitrogen (N2) is not directly usable by plants. During nitrogen fixation, nitrogen-fixing bacteria, such as those found in the root nodules of leguminous plants or free-living in the soil, convert atmospheric nitrogen gas into ammonia (NH3), which plants can then assimilate to form proteins and nucleotides.
It is important to correct some misconceptions; denitrification is the process where denitrifying bacteria convert nitrates (NO3-) to nitrogen gas (N2), and nitrification is the conversion of ammonium (NH4+) to nitrates (NO3-) by nitrifying bacteria, not the other way around. Thus, the correct option for the question is b) Nitrogen fixation.