Final answer:
The term that best describes the process where cells grow on the outside and develop on the inside is differentiation. This involves the specialization of cells to perform distinct functions and is critical during the developmental stages of fetal growth.
Step-by-step explanation:
The process in cells that best describes the concept of things changing outside (growing) and inside (developing) is b) Differentiation. Differentiation refers to the process by which unspecialized cells become specialized in both structure and function. In the context of development, differentiation allows cells to perform specific tasks and is vital for the growth, repair, and proper functioning of multicellular organisms, including the development of specific organs during fetal growth, known as critical periods.
It is important to note that while mitosis is related to the division of cells and the increase in cell number, differentiation specifically refers to the maturation and specialization of these cells.