The correct option is D Meiosis
Meiosis occurs during the reproductive phase and not during the development of the grain. Mitosis in the cells of such root and shoot apical meristems leads to increase in the number of cells. The cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions. This act leading to maturation is termed as differentiation. The plant cells grow in size by cell enlargement which in turn requires water. Turgidity of cells helps in extension growth.