The line within an Earthen dam section below which there is a positive hydrostatic pressure is known as:

Explanation:

Phreatic line or top flow line:

• The top flow line of a saturated soil mass below which seepage takes place is called the Phreatic line.
• The phreatic line is the top flow line that separates the saturated zone from the unsaturated zone and below which positive hydrostatic pressure exists in the dam section.
• Along the phreatic line, atmospheric pressure exists. Its shape is approximately a parabola.
• Hydrostatic pressure acts below the phreatic line whereas atmospheric pressure exists above the phreatic line
• It is not an equipotential line, but a flow line.

Hence option (4) is the correct answer.