In sewage having fully oxidised organic matter, the nitrogen exists in the form of

ANS:B - nitrates

In sewage that has undergone complete oxidation of organic matter, the nitrogen typically exists in the form of nitrates. During the process of nitrification, ammonia (NH3) is converted into nitrites (NO2-) by nitrosomonas bacteria, and then further oxidized into nitrates (NO3-) by nitrobacter bacteria. Nitrates are the end product of this biological process in sewage treatment plants where organic matter has been thoroughly oxidized. Therefore, the correct answer is: b. nitrates