The correct answer is suberin.Concept:Cork: As the plants grow older, the outer protective tissue (i.e., epidermis) undergoes certain changes. A strip of the secondary meristem, called cork cambium replaces the epidermis of the stem.Explanation:Characteristics of cork cells: The cells are rectangular and their protoplast is vacuolated and contains tannins and chloroplasts. Cells are dead and compactly arranged without intercellular spaces.The walls of cork cells are heavily thickened with an organic substance (a fatty substance), called suberin, which makes these cells impermeable to water and gases.The function of cork cells:Cork is protective in function.Its cells prevent desiccation (loss of water from the plant body), infection, and mechanical injury.The cork cambium gives off new cells on both sides, thus, forming cork (phellem) on the outer side and the secondary cortex or phelloderm on the inner side. The layer of cells which is cut by cork cambium on the outer side ultimately becomes several layered thick cork (bark) of trees.Additional Information Lignin: Lignin is an organic polymer that is abundant in the cell walls of some specific cells like sclerenchyma.It has many biological functions such as water transport, mechanical support, and resistance to various stresses.Cutin: It is a waxy water-repellent substance found in the cuticle of plants and is made up of polymerized esters of fatty acids.Cellulose:It is a polysaccharide made up of repeating units of glucose and is found in the cell wall of plants.