What type of chlorine is used to measure the active sanitation level in pool water?

Free chlorine is the correct choice for measuring the active sanitation level in pool water. It represents the portion of chlorine in the water that is available to effectively kill bacteria, algae, and other harmful microorganisms. Free chlorine is not combined with any contaminants or other substances, which means it is readily active and capable of maintaining water quality.

In the context of pool maintenance, monitoring free chlorine levels is crucial because it directly reflects the pool's ability to disinfect the water and ensure safety for swimmers. Pool operators often test for free chlorine to determine if additional chlorine needs to be added to maintain an effective sanitation level.

Combined chlorine, on the other hand, is chlorine that has reacted with contaminants and is no longer effective as a sanitizer. While it's important to monitor both free and combined chlorine levels, free chlorine is the one that indicates active sanitation. Organic chlorine and stable chlorine are terms that are less common in the context of routine pool water testing and do not specifically refer to the active sanitation capability in the same manner as free chlorine.