Right choice is (a) End-of-pipe
The explanation: End-of-pipe is a process which uses free water and air streams of various contaminates at the last stage of the process, most probable number method is used to detect the presence of E. coli which is a fecal coliform bacterium, the chemical testing method detects the presence of ammonia, nitrate, pH or any chemical parameter, dissolved oxygen method measures the level of dissolved oxygen.