The best explanation for what happened is:
a. Molecules of red dye moved by diffusion from a higher concentration to a lower concentration.
This option accurately describes the process by which the molecules of red dye spread throughout the water, moving from an area of higher concentration (where the dye is first added) to areas of lower concentration (the surrounding water), resulting in an even distribution over time.
The other options involve processes (active transport and osmosis) that do not apply to the movement of dye in this context.