Final answer:
Nitrogen fixation is the process through which atmospheric nitrogen is converted to a form usable by living organisms, such as ammonia, through the action of microorganisms employing the enzyme nitrogenase.
Step-by-step explanation:
The correct answer to the question is nitrogen fixation. This process is essential in the conversion of nitrogen in the atmosphere into forms that are usable by living organisms, particularly plants. Nitrogen fixation involves microorganisms that use the enzyme nitrogenase to convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can then be incorporated into organic compounds, aiding in plant growth and development. Many of these microorganisms live symbiotically with plants, such as the rhizobia found in legume root nodules, proving vital in agricultural contexts as well. This conversion is a key component of the nitrogen cycle, which is critical for maintaining the ecosystem's balance by enabling the synthesis of crucial biological molecules like nucleotides and proteins.