The correct answer is: C. always sloping downward in the direction of flow.
The hydraulic grade line (HGL) is a line that represents the total energy of the water in a pipe. It is always sloping downward in the direction of flow, because the water is losing energy as it flows. The energy grade line (EGL) is a line that represents the pressure energy of the water in a pipe. It is always above the HGL, because the pressure energy is always greater than the total energy.
The HGL and EGL are always in the same vertical plane, but they may be offset from each other. The offset is caused by the friction losses in the pipe. The greater the friction losses, the greater the offset between the HGL and EGL.
The HGL and EGL are useful for analyzing the flow of water in pipes. They can be used to determine the pressure at any point in the pipe, and to calculate the head losses in the pipe.