ANS:C - thrice the head loss in the pipe due to friction

For maximum horsepower of a nozzle, the head supplied must be equal to four times the head loss in the pipe due to friction. This concept is derived from the principles of fluid mechanics and is associated with the hydraulic efficiency of the system. When the head supplied (or available) is greater than four times the head loss in the pipe due to friction, there's a diminishing return on the conversion of the available energy into useful work. Therefore, to optimize the horsepower output, the head supplied must be carefully matched with the head loss in the pipe due to friction, ensuring that it is four times greater for maximum efficiency.