The indicator organisms used to determine contamination of drinking water are...

The Coliform group serves as a key indicator for determining contamination in drinking water. These bacteria, which are naturally found in the environment and often in the intestines of warm-blooded animals, are used in water quality testing as a means to evaluate the microbial safety of drinking water. The presence of coliforms signifies potential fecal contamination, which could lead to pathogenic organisms entering the water supply and posing health risks.

Utilizing coliforms as indicators is effective because they are easier and less expensive to detect than many pathogens, providing a reliable indication of the overall sanitation quality of the water. If coliforms are present, it raises concerns that there may also be dangerous bacteria or viruses that can cause illness.

Other organisms mentioned in the choices may indicate specific types of contamination, but they do not serve broadly as indicators for general water safety. For instance, iron bacteria can cause aesthetic issues but are not indicative of fecal contamination. Giardia lamblia, while a pathogen found in fecal material, is not as easily detectable in routine water testing as coliforms. Salmonella is also a specific pathogen and does not represent the general presence of fecal matter as effectively as the coliform group does. Thus, coliforms are recognized as a