Why does active transport require ATP?

A. An input of energy is needed to both speed up the rate of facilitated diffusion and maintain the conformation of transport proteins.
B. An input of energy is needed to allow the movement of molecules from an area of low concentration to one of higher concentration.
C. An input of energy is needed to maintain the conformation of transport proteins.
D. An input of energy is needed to speed up the rate of facilitated diffusion.

In Active transport, an input of energy in the form of ATP is needed to transport substances against the concentration gradient, I. E it allow the movement of molecules from an area of low concentration to one of higher concentration. And hence it is called active transport because energy is required to drive this process.

Active transport is the opposite of passive transport, where molecules move from a region of higher concentration to a region of lower concentration without the input of ATP.