It is a. epidermal cells B. mesophyll cells C. guard cells D. veinlets in plants that control the opening and closing of stomata and regulate water transpiration.

Analysis of test questions: Stomata is the portal of transpiration and water loss of plants, surrounded by two half-moon guard cells. Guard cells regulate transpiration by opening and closing stomata. Therefore, it is not epidermal cells that control the opening and closing of stomata and regulate the transpiration of water in plants, so A is wrong; Not mesophyll cells, so b is wrong; But to defend cells, so c is correct; It's not a venous cell, so d is wrong.