Final answer:
Combined chlorine is obtained by subtracting the free chlorine from the total chlorine. Free chlorine consists of disinfecting agents, while combined chlorine includes reduced forms that no longer disinfect.
Step-by-step explanation:
Combined chlorine can be calculated by subtracting the measured value for free chlorine from the measured value for total chlorine. The free chlorine residual includes forms of chlorine such as Cl₂, HOCl, and OCl⁻, which are available for disinfecting the water supply. On the other hand, the combined chlorine residual includes species in which chlorine is in its reduced form, such as NH₂Cl, NHCl₂, and NCl₃. These forms of combined chlorine are no longer capable of providing disinfection. Hence, the correct answer to the question is C) Total Chlorine.