The correct answer is: B. water is under pressure between two impervious strata.
An artesian aquifer is a confined aquifer where groundwater is under pressure and rises to the surface or to a level above the top of the aquifer without pumping. The pressure is caused by the weight of the water and overlying material above the aquifer. The water in an artesian aquifer is typically clear and cold, and it can be used for drinking, irrigation, and other purposes.
Option A is incorrect because the water surface under the ground is not always at atmospheric pressure in an artesian aquifer. The water can be under pressure if it is confined between two impervious strata.
Option C is incorrect because the water table is not always the upper surface of the zone of saturation in an artesian aquifer. The water table can be below the upper surface of the zone of saturation if the aquifer is confined.
Option D is incorrect because artesian aquifers do exist.