Explanation
During secondary growth in a stem, the cork cambium cuts off cells on its outer and inner sides. The outer cells differentiate into phellem, also known as cork cells. Therefore, the correct option is A: Phellem.
Phellogen is another name for cork cambium, which is the meristematic tissue responsible for producing the cork cells. Phelloderm is a tissue that forms on the inner side of the cork cambium and is derived from the inner cells of the cork cambium. Periderm is a collective term that refers to all the tissues that are produced by the cork cambium, including phellem, phellogen, and phelloderm.
