ANS:B - 6.4 m

To prevent vaporization in pipe lines, they are laid over the ridge so that they are above the hydraulic gradient line but not more than a certain height. This prevents the liquid from reaching a pressure low enough to cause boiling or vaporization. Typically, pipe lines are laid over the ridge so that they are not more than 6.4 meters above the hydraulic gradient line. This limitation ensures that the pressure remains above the vapor pressure of the liquid being transported, thus preventing vaporization. Therefore, the correct answer is 6.4 meters.