Concept:Enzymes are biological catalysts that are chemically composed of proteins made from amino acids.Explanation:Enzymes are high‑molecular‑weight proteins. Amino acids linked by peptide bonds form their structure. They speed up reactions by lowering activation energy. Enzymes can be denatured by heat, salts, or solvents – a typical protein property. Other options: lipids are fats and oils; acids donate protons or hydrogen ions; carbohydrates are sugars and starches. None of these match the chemical nature of enzymes. Hence, enzymes are chemically proteins.Answer:D. protein