Hydraulic gradient line (H.G.L.) represents the sum of pressure head and datum head.

Explanation:
The hydraulic gradient line (H.G.L.) is a graphical representation that shows the total energy of a fluid flowing in a pipe or channel. It is a line that connects the water surface in a reservoir or a water body to the hydraulic grade line (H.G.L.) at any point in a pipe or channel. The hydraulic grade line (H.G.L.) represents the sum of the pressure head and the datum head.

1. Pressure Head:
Pressure head is the energy per unit weight of fluid due to the pressure exerted on it. It is calculated as the pressure divided by the unit weight of the fluid. The pressure head is directly proportional to the pressure and represents the potential energy of the fluid.

2. Datum Head:
Datum head is the energy per unit weight of fluid at a reference elevation or datum level. It is the reference point from which the elevation of other points in the system is measured. The datum head is constant throughout the system and represents the potential energy of the fluid at the reference level.

3. Hydraulic Grade Line (H.G.L.):
The hydraulic grade line (H.G.L.) is the sum of the pressure head and the datum head. It represents the total energy of the fluid in a pipe or channel at any point. The H.G.L. is a graphical representation that helps in analyzing and designing fluid flow systems. It helps in determining the pressure and energy losses in the system.

By adding the pressure head and the datum head, we get the hydraulic grade line (H.G.L.). This line represents the total energy of the fluid and helps in understanding the flow characteristics and energy distribution in the system. The H.G.L. is used in various engineering applications, such as water supply systems, irrigation systems, and hydraulic structures.

In conclusion, the correct answer is option 'D', which states that the hydraulic gradient line (H.G.L.) represents the sum of the pressure head and the datum head.