The correct option is C Both a and b
To perform certain functions, cells undergo different structural changes like change in the cell walls and protoplasms. For example - the development of very strong, elastic, and lignocellulosic elements to carry the water to long distances. This process is called differentiation. Parenchyma cells are an example of differentiated cells.
Now, these differentiated cells lose the ability to divide, but under certain conditions, these cells again regain their ability to actively divide. This process is termed dedifferentiation. For example - interfascicular cambium and cork cambium from fully differentiated parenchyma cells.