The best answer is (3).
Guard cells help regulate water loss by controlling the opening and closing of stomata which facilitates gas exchange and transpiration in plants.
Guard cells are able to control the opening and closing of stomata by changing shape. The guard cells take in potassium ions which causes water to diffuse into them by osmosis. The cells then swell and become bowed. When this happens, they bend way from each other thereby opening the stomata.
When they lose the potassium ions, water diffuses out by osmosis and the cells become flaccid and less bowed thus closing the stomata between them.
