The answer is D, digestion.
In digestion, large complex insoluble food is digested into small, simple, soluble molecules that then can be absorbed by the small intestine.
Digestion usually involves different enzymes which includes amylase that digests starch, lipase that digests fats, protease that digests protein and more.
Digestion starts in our oral cavity when starch is digested. Though they won't be completely digested as it continues in duodenum. Aside from checmicals digestion which evolves enzyme, there's also physical digestion that breaks the molecules into smaller pieces to speee up chemical digestion later on.