The correct answer is A) diffusion.
Diffusion is the process where molecules spread from an area of high concentration to an area of low concentration until they are evenly distributed. In the case of a drop of food coloring in water, the food coloring molecules start at a high concentration in the drop and gradually move into the surrounding water, creating an even distribution of color. This movement continues until the concentration of coloring is uniform throughout the cup, which is the essence of diffusion.