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Enzymes are mostly proteins (some RNAs act as ribozymes) that function as biological catalysts. They increase reaction rates by lowering the activation energy, allowing reactions to proceed efficiently at the temperatures and pH ranges found in living systems. They exhibit specificity for substrates, often via an active site whose shape and charge complement the substrate (lock-and-key or induced-fit models). Option C captures these essentials. Option A confuses enzymes with lipids, which mainly store energy and compose membranes. Option B describes nucleic acids (DNA/RNA) and genetic information, not catalytic function (except special ribozymes). Option D reduces carbohydrates to structure only; while cellulose and chitin are structural, carbohydrates also serve energy roles—but they aren’t enzymes. For GED Science, pair macromolecules with core functions: proteins = enzymes/structure/transport; nucleic acids = information; lipids = membranes/long-term energy; carbohydrates = energy/structure. Recognizing activation energy and specificity keywords is a quick route to correct enzyme questions.