Which of the following is considered a disinfectant?

Chlorine is considered a disinfectant because it is used to kill or inactivate harmful microorganisms in water, making it safe for consumption and use. It works by disrupting the cellular processes of bacteria, viruses, and other pathogens, effectively reducing their numbers and preventing disease transmission. Chlorine is widely utilized in water treatment facilities to ensure that drinking water meets safety standards.

In contrast, the other options do not serve the primary purpose of disinfection. A bacterium refers to a broad category of microorganisms, some of which can be harmful but do not serve as disinfectants themselves. Aluminum sulfate is primarily used as a coagulant in water treatment processes, helping to remove particulate matter from water rather than disinfecting it. A sand filter is a physical filtration method that removes sediments and some pathogens from water but does not actively kill microorganisms like a disinfectant does. Thus, chlorine's specific role in actively providing disinfection makes it the correct choice.