Which of the following is a requirement for facilitated diffusion?

Facilitated diffusion specifically requires that the movement of substances occurs along their concentration gradient, meaning that molecules move from an area of higher concentration to an area of lower concentration. This process relies on the presence of specific carrier proteins or channels in the cell membrane that assist in the transport of certain molecules, like glucose or ions, without expending energy. Since facilitated diffusion does not require energy and takes advantage of the natural tendency of molecules to move towards equilibrium, the movement direction aligning with the concentration gradient is essential for this transport mechanism.

The other options are not accurate as they suggest mechanisms or properties that are inconsistent with the nature of facilitated diffusion. For instance, the idea of moving against the concentration gradient would imply active transport, which requires energy input, rather than the passive process of facilitated diffusion. Similarly, the concept of a carrier molecule being able to transport any molecule or being non-specific does not accurately reflect the selective nature of the membrane proteins involved in this form of transport. The specificity of the carrier proteins is crucial for the correct functioning of facilitated diffusion.