The correct answer is "Differentiation".
Differentiation is a process by means of which cells start to form groups and become specialized to perform a particular task. The process of differentiation increases the efficiency of a tissue to accomplish a particular task. Due to differentiation, the structure and shape of cells also changes according to the type of work they are destined to do. It happens in higher organisms where complex body structures exist. Cells differentiate to form tissue and start to show division of labor. A group of tissues further forms organ & organs form organ system. An organ system is specialized to perform only one task with great efficiency. For example, in plants, cells form a group to become xylem tissue which is specialized in only transport of water and does not indulge in any other work.