A. It is the process by which a substance requires energy to pass through a membrane against a concentration gradient: This describes active transport, not diffusion. Active transport requires ATP to move substances against their gradient. B. It is the process by which solvent molecules pass through a semipermeable membrane from a less concentrated solution into a more concentrated one: This describes osmosis, not diffusion. Osmosis specifically refers to the movement of water. C. It is the process by which molecules move from high-concentration areas to low-concentration areas: This correctly defines diffusion - the passive movement of molecules down their concentration gradient. D. It is the process by which certain molecules pass through the plasma membrane with the help of carrier proteins: This describes facilitated diffusion, which still moves substances down their gradient but requires a protein carrier.