The seepage force in a soil, is

ANS:D - all the above.

F = (unit weight of water)x h
F = (unit weight of water)×(h/L).L
F = i.L.(unit weight)
F is directly proportional to "i". Seepage occur due to head difference. If head is more seepage is more. And if head is more then hydraulic gradient also more (i=h/l). Seepage force acts along flow lines. i.e When flow occurs from u/s to d/s below the base of the foundation or structure. So the opt A is correct also B and C too. Equipotential lines are perpendicular to flow lines and than the seepage occur below the dam, the see page flow parallel to that of flow lines exit gradient = dh/dl.