Which of the following is a disinfectant form used in water treatment that is a gas at room temperature?

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Disinfection in water treatment can use different chemical forms, but a true gas at room temperature that’s used directly as a disinfectant is chlorine gas. It’s stored and fed as a gas and dissolves in water to form hypochlorous acid, providing rapid disinfection and leaving a residual in the distribution system. The other options are not in gaseous form at room temperature or are typically used in aqueous form: hypochlorite is a liquid solution, chloramine is formed and used in water as a chlorinated aqueous compound, and ozone, while a gas, is usually generated on-site and dissolved for disinfection. So chlorine gas best fits the description of a disinfectant form that is a gas at room temperature.