Option D - Nitrogen fixation is a process that changes plant and soil Ammonia into nitrogen.
Explanation
Nitrogen fixation is a chemical process in plant which of nitrogen in the soil that plants converted into ammonia and nitrogenous compounds. This is mainly food source for plants under microscopic view. The atmospheric nitrogen is useless and relatively it is non-reactive. Nitrogen fixation is beneficial for plants as it the important source of nitrogen which is required for the soil and for plants growth.
