The phreatic line is the boundary between the saturated and unsaturated zones in the ground. It represents the water table, which is the level at which the soil or rock is completely saturated with water. Determining the location of the phreatic line is important in various civil engineering projects such as designing foundations, excavations, and groundwater management.

There are several methods used to locate the phreatic line, and the correct answer is option 'D', which states that all of the mentioned methods can be used. Let's discuss each method in detail:

a) Graphical Method:
The graphical method involves plotting the water table elevations at various locations on a graph or map. This can be done by using data from monitoring wells or piezometers. By connecting the points of equal elevation, a contour map of the water table can be created. The phreatic line can then be identified as the contour line that represents the boundary between the saturated and unsaturated zones.

b) Experimental Method:
The experimental method involves conducting field tests to directly measure the water table elevation. One common technique is the installation of monitoring wells or piezometers, which are instruments used to measure the groundwater level. By measuring the water level in these wells at different locations, the elevation of the phreatic line can be determined.

c) Analytical Method:
The analytical method involves using mathematical equations and models to estimate the water table elevation. This method relies on various factors such as soil properties, groundwater flow, and boundary conditions. Analytical solutions can be derived from groundwater flow equations, and these solutions can be used to estimate the position of the phreatic line.

By using a combination of these methods, engineers and hydrologists can accurately locate the phreatic line and understand the behavior of groundwater in a particular area. It is important to note that the choice of method depends on the site conditions, available data, and project requirements.