ANS:A - nitrates

In a well-oxidized sewage, nitrogen is primarily present in the form of nitrates (NO3^-). During the process of sewage treatment, organic nitrogen compounds are broken down by microorganisms through nitrification and denitrification processes. In the nitrification process, ammonia (NH3) is oxidized to nitrites (NO2^-) and then further oxidized to nitrates (NO3^-). Nitrates are the end product of this oxidation process and are relatively stable under aerobic conditions. So, the correct option is: "nitrates."