Chlorine is available to kill bacteria under which condition?

Chlorine is most effective at killing bacteria when it is available as free chlorine in the water. This availability is influenced by several factors, including the concentration of chlorine in the water and the presence of contaminants. When chlorine is added to water, it begins to dissociate, forming hypochlorous acid and hypochlorite ions, both of which are capable of inactivating pathogens.

While breakpoint chlorination is an important process to ensure all combined chlorine is converted to free chlorine, it is not a requirement for chlorine to be effective immediately upon addition. Therefore, chlorine does have the capability to kill bacteria as soon as it is introduced into water, assuming it is in an appropriate concentration and there are no overwhelming contaminants that would necessitate a higher concentration or specific conditions.

The statements regarding only killing bacteria at levels above 10 ppm, or implying chlorine is only effective at breakpoint chlorination, do not encompass the full operational dynamics of chlorine as a disinfectant in pool water. Thus, the initial immediate impact of chlorine upon its addition is what primarily supports this option as the correct answer.