Explanation:

Chlorine ($\text{Cl}_2$) is the most widely used chemical disinfectant for water purification globally due to its effectiveness in killing bacteria and viruses, its relatively low cost, and its ability to maintain a residual disinfectant level in the water distribution system, preventing recontamination. - Option A (Propane): Propane ($\text{C}_3\text{H}_8$) is a hydrocarbon gas primarily used as fuel; it has no role in water disinfection. - Option B (Carbon dioxide): Carbon dioxide ($\text{CO}_2$) is used in water treatment for pH adjustment (recarbonation) but not as a primary disinfectant for pathogens. - Option C (Chlorine): Chlorine is highly effective and widely used for disinfecting drinking water by oxidizing and destroying microorganisms. This is the correct answer. - Option D (Oxygen): Oxygen ($\text{O}_2$) is naturally present in water and is involved in aeration processes to improve taste and odor, but it is not a primary disinfectant for killing pathogens.