When a nutrient moves freely across the cell membrane from an area of higher nutrient concentration into an absorptive cell where the concentration of that nutrient is lower, this is

diffusion

Explanation:

Diffusion is the over all movement of molecules from one region where it is in high concentration to another another region where it is in lower concentration.

This movement continues until the concentration of both regions are equal. Smaller non polar molecules are able to diffuse through the lipid bilayer of the membrane .

Movement through the membrane is dependent on the concentration gradient, that is the difference in concentration between the two areas.

It is also dependent on the size of the molecule

the answer to your question is:

c. complementary base-pairing