Diffusion is the process of the movement of solute particles on the basis of the concentration gradient.
During homeostasis or the process of maintaining the internal environment of the body, the nutrients which could be the molecules or ions move from the region of high concentration or amount to a region of their low concentration through the cell membrane.
The concentration of calcium inside the cells during homeostasis is maintained by the diffusion of the calcium ions. Thus, option-D is the correct answer.
