The correct answer is: A. free chlorine
Free chlorine is the amount of chlorine that is not combined with other substances in the water. It is the most effective form of chlorine for disinfection, and it is used to kill bacteria and other microorganisms in drinking water.
Residual chlorine is the amount of chlorine that remains in the water after disinfection. It is important to have a residual chlorine level in drinking water to prevent recontamination.
Combined available chlorine is the amount of chlorine that is combined with other substances in the water, such as ammonia. It is not as effective as free chlorine for disinfection, but it can still help to kill bacteria and other microorganisms.
Here is a brief explanation of each option:
- Free chlorine is the amount of chlorine that is not combined with other substances in the water. It is the most effective form of chlorine for disinfection, and it is used to kill bacteria and other microorganisms in drinking water.
- Residual chlorine is the amount of chlorine that remains in the water after disinfection. It is important to have a residual chlorine level in drinking water to prevent recontamination.
- Combined available chlorine is the amount of chlorine that is combined with other substances in the water, such as ammonia. It is not as effective as free chlorine for disinfection, but it can still help to kill bacteria and other microorganisms.