Which of the following chemicals can be used for pool disinfection?

Iodine is a chemical that can be used for pool disinfection, particularly in situations where chlorine-based disinfectants might not be suitable. It is effective at killing a broad spectrum of pathogens, including bacteria and viruses, making it a viable alternative for sanitizing water.

In contrast, ammonia is primarily used for cleaning and does not serve as a suitable disinfectant for pool water due to its potential to react with chlorine and form chloramines, which can exacerbate water quality issues. Soda ash is mainly employed to raise pH levels in pool water; it does not possess any disinfectant properties. Sodium thiosulfate is typically used as a dechlorinating agent and is not intended for disinfection purposes.

Thus, the correct choice highlights iodine's role as an effective disinfectant option for pools under certain conditions.