Which process occurs in the structures that are labeled X?

According to a source, mitosis.

Mitosis and meiosis are simply cell division processes that occurs differently, they're characteristically divergent from each other according to their function and structure. Mitosis is the cell division that happens in all cells in the human body except sperm and egg cells. They produce diploid cells. Take for instance your integumentary system, layer of the skin in which your stratum basale always produces new epithelial cells (via mitosis) to take over until the outer layer, called stratum corneum (a continous replaced dead cells in this layer).