Which of the following gas or gases are used to disinfect water in sewage treatment facilities?

Both ozone and chlorine are used. Chlorine (as gas or hypochlorite) is the cheapest and most common disinfectant, applied in a contact tank before the treated water is discharged; ozone, generated on site, is a stronger oxidant that destroys bacteria and viruses quickly and leaves no chlorinated by-products. Nitrogen is an inert gas with no disinfecting action, so any choice containing it is wrong, and naming chlorine alone leaves out ozone.