Cork (phellem) is not considered as living tissue. It is a water-resistant, protective tissue that displaces the stem epidermis during the secondary growth phase. A mature cork cell is non-living and has cell walls that are composed of a waxy substance that is highly impermeable to gases and water known as suberin. Generally, the phloem together with the cork cells are considered to comprise the outer bark. The epidermis is a single-layered group of cells that covers plant leaves, flowers, roots, and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds and absorbs water and mineral nutrients. Cortex and pericycle are not protective tissues.
Thus, the correct answer is option D.