The correct answer is: A. 2.4 m above the hydraulic gradient.
The hydraulic gradient is the slope of the water table. The water table is the surface below which the ground is saturated with water. The hydraulic gradient is important because it determines the flow of water in a pipe. If the pipe is laid above the hydraulic gradient, water will flow out of the pipe and into the ground. This can cause the pipe to become empty and can also lead to problems with water quality.
The other options are incorrect because they are too high. If the pipe is laid too high above the hydraulic gradient, water will not flow into the pipe and the pipe will not be able to carry water.
To avoid vaporisation in the pipe line, the pipe line over the ridge is laid such that it is not more than 2.4 m above the hydraulic gradient. This will ensure that the pipe is always full of water and that there is no risk of vaporisation.