At pH values above 7.5, the disinfectant capability of chlorine is

Above pH 7.5 the disinfecting power of chlorine is greatly reduced.

It is not greatly enhanced, since higher pH shifts chlorine to its weaker form.

Nor is it slightly enhanced, as the trend is a clear decline in effectiveness.

It does change with pH, so remaining unchanged is not correct.

At higher pH chlorine exists more as hypochlorite ion, a far weaker disinfectant than hypochlorous acid.

Water treatment therefore favours slightly acidic to neutral pH for effective chlorination.