The correct answer is B. Nitrates.
Nitrates are the most oxidized form of nitrogen and are the end product of the nitrogen cycle. They are formed when ammonia is oxidized by bacteria in the presence of oxygen. Nitrates are essential for plant growth, but they can also be harmful to humans and animals if consumed in high levels.
Nitrites are the intermediate form of nitrogen between ammonia and nitrates. They are formed when ammonia is oxidized by bacteria in the absence of oxygen. Nitrites are toxic to humans and animals, and they can cause methemoglobinemia, a condition in which the blood is unable to carry oxygen.
Free ammonia is the least oxidized form of nitrogen. It is formed when organic nitrogen is broken down by bacteria. Free ammonia is toxic to humans and animals, and it can cause respiratory problems.
Aluminoid nitrogen is a form of nitrogen that is bound to aluminum. It is not a major form of nitrogen in sewage.
In sewage having fully oxidised organic matter, the nitrogen exists in the form of nitrates. This is because the nitrates are the most oxidized form of nitrogen and are the end product of the nitrogen cycle.