The best description of the type of cellular transport that the student is investigating is:
D. The student is investigating the passive transport of diffusion.
In this scenario, the movement of red dye from an area of higher concentration (where the dye is dropped) to an area of lower concentration (throughout the water) is an example of diffusion, which is a passive transport process (meaning it does not require energy). Osmosis specifically refers to the movement of water across a selectively permeable membrane, which is not the focus here.