The ascent of xylem sap in plants is mainly accomplished by the 

The ascent of xylem sap in plants is mainly accomplished by the cohesive and adhesive properties of water molecules. Cohesion refers to the attraction between water molecules, allowing them to form a continuous column in the xylem vessels. Adhesion refers to the attraction between water molecules and the cell walls of the xylem vessels, helping to counter the force of gravity and allowing water to rise through
the plant from the roots to the leaves.
(1) The size of the stomatal aperture in the leaves primarily regulates the exchange of gases (CO2 and O2) and
water vapour during transpiration. While transpiration is an essential process for the movement of water through plants, it is not the main factor responsible for the ascent of xylem sap.
(2) The distribution of stomata on the upper and lower epidermis of leaves is important for controlling gas exchange and water loss during transpiration. However, it is not the primary factor responsible for the ascent of xylem sap. 
(4) Root pressure is the pressure exerted by the roots of some plants that can push water up the xylem vessels. While root pressure can contribute to the ascent of xylem sap in certain situations, it is not the main  mechanism responsible for long-distance water transport in most plants.