Final answer:
Active transport requires energy to move substances from a lower to a higher concentration, while facilitated diffusion moves substances from a higher to a lower concentration without energy input. Active transport goes against the concentration gradient ([S]low to [S]high), as noted in option C of the question.
Step-by-step explanation:
The primary difference between active transport and facilitated diffusion is that active transport requires energy to move substances against their concentration gradient, meaning from [S]low to [S]high concentration, which is represented by choice C) the direction of transport is [S]low → [S]high. In contrast, facilitated diffusion does not require energy, as it involves the movement of substances from areas of higher concentration to areas of lower concentration.
Facilitated diffusion can also be distinguished by its characteristic saturation point, akin to enzyme kinetics, where the rate of transport reaches a maximum when all transporter proteins are occupied, whereas active transport maintains its ability to move substances regardless of concentration differences as long as there is energy available.