Concept:Biotechnology uses living organisms, like microbes, to carry out useful environmental processes.
Explanation:Biotechnology is applied mainly in biological sewage treatment.
Microbes are used to break down organic waste present in sewage.
Common biological methods include activated sludge process, oxidation ponds, trickling filters and anaerobic treatment.
These biological methods are not used for water treatment, chemical purification or metal purification.
Hence, the correct field of application is sewage treatment.
Answer:A. treatment of sewage