The correct option is C Nitrogen fixation
Nitrification is a microbial process by which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrite and nitrate.
Denitrification is a microbially facilitated process where nitrate is reduced and ultimately produces molecular nitrogen (N2).
Nitrogen fixation is a process by which nitrogen in the Earth's atmosphere is converted into ammonia (NH3) or other molecules available to living organisms.
The Haber process, is an artificial nitrogen fixation process and is the main industrial procedure for the production of ammonia.
Thus, the correct answer is option (C).