Denitrification occurs under: anaerobic conditions aerobic conditions both aerobic and anaerobic conditions

Denitrification is the process in which the nitrogen compounds makes its way back into the atmosphere by converting nitrate into gaseous nitrogen. Denitrification is a microbially facilitated process where nitrate (NO3–) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Denitrification is carried out by the denitrifying bacterial species i.e, Clostrdium & Pseudomonas. Facultative anaerobic bacteria –Facultative anaerobes are bacteria that can grow in both the presence or absence of oxygen.Denitrification is largely an anaerobic process, but can occur aerobically as well under some conditions. Facultative anaerobic bacteria perform denitrification as a type of respiration that reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter.The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO3−), nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O) finally resulting in the production of dinitrogen (N2). Denitrification is an anaerobic process which converts nitrate to dinitrogen in the following sequence: NO3-→ NO2- → NO → N2O→ N2 Thus, Denitrification occurs under anaerobic. Option 3rd i.e, both anaerobic and aerobic will not correct because this option says some steps will aerobic and some steps will anaerobic, while this does not occur. In some conditions denitrification can occur aerobic not in steps of denitrification.