The answer is A, as most sewage treatment plants use bacteria to break down and remove nutrients. The main water pollution problem caused by sewage effluent is eutrophication, related to the release of nutrients into rivers and lakes. Nutrients such as nitrogen and phosphorus ions result in high algal growth. Some algal blooms release toxins, and algal blooms also disrupt the natural dissolved oxygen concentration cycles. Fish kills are often because eutrophication has resulted in anoxic conditions.
Anyway, wastewater treatment plants utilise bacteria to facilitate nitrification or denitrification, which removes ammonia and nitrates/nitrites, respectively.