Which of the following compounds can indicate the presence of organic contaminants in water?

Activated carbon is a compound that is widely used in water treatment processes, particularly for its ability to adsorb organic contaminants. This material functions effectively by trapping organic molecules within its porous structure, thus removing them from the water. The presence of organic contaminants can often be indicated by the need to use activated carbon as a filtration medium, as these substances tend to be volatile and can carry various pollutants that impact water quality.

While ozone is a powerful oxidizer and is used for disinfection and odor control, it does not directly indicate the presence of organic contaminants; instead, it acts on them to break down organic substances. Chlorine is primarily used for disinfection and can react with organic materials, but it is not a direct indicator of their presence either. Magnesium, meanwhile, is a naturally occurring mineral and does not provide information about organic contaminants in water. Instead, it is more associated with water hardness. Therefore, activated carbon is the most relevant choice when it comes to detecting and indicating the presence of organic contaminants in water supplies.