Explanation:

Chlorine is the most widely used and effective disinfectant for water purification globally. It is used in various forms, such as chlorine gas ($Cl_2$), sodium hypochlorite ($NaClO$), or calcium hypochlorite ($Ca(ClO)_2$), to kill bacteria, viruses, and other microorganisms present in water, making it safe for consumption. Option (A) Propane is a fuel gas and has no disinfectant properties. Option (B) Carbon dioxide is a greenhouse gas and is not used as a disinfectant. Option (D) Oxygen, while essential for life, is not used as a primary disinfectant in water treatment, although ozone (an allotrope of oxygen) is a powerful disinfectant, it is not simply 'oxygen'. Therefore, Chlorine is the correct answer.