Enzymes are mostly proteins

Steps:

• Recall that enzymes speed up reactions as biological catalysts.
• Identify the main biomolecule for enzymes: proteins fold into specific 3D shapes for catalysis.
• Eliminate non-protein options by their primary roles in biology.
• Confirm proteins match the enzyme function via their amino acid structure.

Why D is correct:

• Enzymes are proteins, as defined in biochemistry: polypeptides with active sites that lower activation energy for reactions.

Why the others are wrong:

• A. Carbohydrates store energy or provide structure, not catalysis.
• B. Fats store energy and form membranes, lacking catalytic sites.
• C. Nucleotides build DNA/RNA for genetic info, not enzyme activity.

Final answer: D