Drawing a flow net is an essential step in solving problems related to seepage through soil. A flow net provides a graphical representation of flow lines and equipotential lines, which help in understanding the flow patterns and calculating quantities such as seepage velocity and discharge. In order to draw a flow net, the following essentials are required:

a) Top Flow and Phreatic Line:
The top flow line represents the free water surface above the soil, while the phreatic line represents the water table within the soil. These lines are important in defining the boundaries and constraints of the flow net. The top flow line is usually horizontal and parallel to the base of the soil, while the phreatic line follows the path of least resistance.

b) Streamlines:
Streamlines are imaginary lines that represent the flow direction of water within the soil. They are drawn perpendicular to the equipotential lines and are used to determine the flow pattern and paths. Streamlines do not intersect each other and are evenly spaced.

The flow net consists of a series of flow channels bounded by streamlines and equipotential lines. The flow channels represent zones of constant flow and are used to calculate the flow rate through the soil.

c) Equipotential lines:
Equipotential lines are imaginary lines that connect points of equal hydraulic potential. They are drawn parallel to the direction of flow and help in understanding the variation in head or pressure within the soil. Equipotential lines intersect streamlines at right angles.

The spacing between equipotential lines represents the hydraulic gradient and can be used to calculate the seepage velocity. Closer spacing indicates a steeper gradient and faster flow.

d) All of the mentioned:
While options a) and b) are the most essential elements required to draw a flow net, it is important to note that option d) "All of the mentioned" is also correct. This is because both streamlines and equipotential lines are necessary to accurately represent the flow patterns and calculate the relevant quantities. However, if we strictly consider the bare minimum essentials, option a) "Top Flow and Phreatic Line" is sufficient.

In conclusion, the essentials required to draw a flow net include the top flow and phreatic line, which define the boundaries, and streamlines and equipotential lines, which represent the flow direction and hydraulic potential within the soil.