Which of the following is NOT commonly used to disinfect water?

Silver nitrate is not commonly used to disinfect water due to its limited effectiveness and practicality compared to other disinfectants listed. While silver has antimicrobial properties and can inhibit the growth of bacteria, it is not a primary choice for water disinfection on a large scale. This is largely due to the cost and complexity of using silver nitrate in comparison to more widely accepted disinfectants like chlorine, chlorine-ammonia compounds, and ozone.

Chlorine is favored for its broad-spectrum efficacy, ease of use, and cost-effectiveness, making it a staple in water treatment facilities. Chlorine-ammonia compounds, used in the form of chloramines, provide a stable and long-lasting disinfectant that reduces the risk of pathogen regrowth in pipes. Ozone is another powerful disinfectant known for its ability to inactivate a wide range of microorganisms effectively. However, ozone requires careful handling and has regulatory considerations due to its potential health effects when not managed properly.

Therefore, in the context of this question, silver nitrate stands out as the least common choice for routine water disinfection, making it the correct answer.