The correct answer is option 2: Endosmosis. Stomatal opening is the process by which the stomata, small openings on the surface of leaves, allow for the exchange of gases and the regulation of transpiration. When the guard cells that surround the stomatal pore gain water through the process of endosmosis, they become turgid and swell. This swelling results in the opening of the stomatal pore, allowing for the exchange of gases such as carbon dioxide and oxygen with the surrounding environment. Option 1, exosmosis, is the movement of water out of cells, which would not result in stomatal opening. Option 3, plasmolysis in guard cells, refers to the process of the cells losing water and shrinking, which would close the stomatal pore. Option 4, a decrease in concentration of cell sap, would not directly influence the process of stomatal opening.