The correct option is A division of differentiated cells
Dedifferentiation is the process in which differentiated cells (cells that have matured to perform a specific function) regain the capacity to divide under certain conditions. For example, the differentiated parenchyma cells (not actively dividing cells) undergoes despecialisation to form interfascicular cambium and cork cambium. This ability to undergo dedifferentiation in plant cells is even used for cell culture experiments.
Senescence is the complete loss in the power of cell division and is generally associated with aging, but in dedifferentiation, cells do undergo division.
Dedifferentiation is also not defined as the growth in the existing cell size.