Answer:
The correct answer will be option D-concentration gradient.
Step-by-step explanation:
Passive transport is the movement of material which does not require energy in the form of ATP.
This mode of transport is largely dependent on the concentration gradient of the molecule that is move from a region of high concentration to low concentration. It is of three types in the cell: diffusion, osmosis and facilitated diffusion.
Thus, option D is the correct answer.