Concept:Active transport is an energy-requiring process that moves substances against the concentration gradient, while diffusion is passive and moves substances along the gradient.Explanation:Diffusion and osmosis are passive processes; they move substances from a region of higher concentration to a region of lower concentration.Active transport, however, uses energy (ATP) to move substances from a lower concentration to a higher concentration, that is, against the concentration gradient.This is the key difference between active transport and diffusion.Answer:B. allows the movement of substances against concentration gradient.