Which of these cells of the dermal tissue of plants prevent water loss and control gas exchange? a. phloem cells b. companion cells c. parenchyma cells d. guard cells

The answer is guard cells. 
Guard cells are cells surrounding each stoma. They help regulate the rate of transpiration by opening and closing the stomata. They are specialized cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore (stoma).

Bis the correct answer.

Because people are wasting it