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

Bromine is not commonly used to disinfect water, which makes it the correct choice in this context. Chlorine is widely recognized for its effectiveness in water disinfection processes, such as in municipal water systems and swimming pools. It is appreciated for its ability to kill a broad spectrum of pathogens, including bacteria and viruses. Iodine is another well-known disinfectant that is particularly effective in smaller applications, such as treating water during emergencies or for individual use, like in camping scenarios. Fluorine, while it has applications in water treatment, is primarily used for the purpose of fluoridation to prevent dental cavities rather than as a disinfecting agent.

Bromine, on the other hand, is more commonly used in certain specific contexts, like hot tubs or spas, due to its stability at higher temperatures but is not standard practice for general water disinfection, especially in public health contexts. Therefore, while bromine does have its place in the treatment of water in niche applications, it lacks the widespread use that chlorine, iodine, and to some extent, fluorine have in the public water supply systems.