a. True
Cellular differentiation occurs throughout the cell development of a multi-cellular organism. It occurs when the cell changes from a simply zygote into a complex system of tissues and cell types.
Stems cells are cells which have the potential to develop into many different cell types in the body during early life and growth. Stem cells serve as internal repair system in many tissues. When stem cells divide while undergoing cell differentiation, it can either retain being a stem cell or become another type of cell like muscle cell, brain cell, or red blood cell.