The correct answer is: C. Sum of applied and residual chlorine.
Chlorine demand is the amount of chlorine that must be added to water to achieve a desired level of disinfection. The applied chlorine is the amount of chlorine that is added to the water, and the residual chlorine is the amount of chlorine that remains in the water after a period of time. The sum of the applied and residual chlorine is the total chlorine demand of the water.
Option A, applied chlorine, is the amount of chlorine that is added to the water. This is not the same as the chlorine demand, because some of the applied chlorine may react with other substances in the water and be removed.
Option B, residual chlorine, is the amount of chlorine that remains in the water after a period of time. This is not the same as the chlorine demand, because some of the applied chlorine may react with other substances in the water and be removed.
Option C, sum of applied and residual chlorine, is the total amount of chlorine that is added to the water and that remains in the water after a period of time. This is the same as the chlorine demand.
Option D, difference of applied and residual chlorine, is not the same as the chlorine demand. This is because some of the applied chlorine may react with other substances in the water and be removed.