Which statement best describes proteins?

• A. They are simple sugars.
• B. They are lipids that store energy.
• C. They are nitrogenous organic compounds consisting of long chains of amino acids; they are essential for structure.
• D. They are inorganic salts.

Answer: (C)

Proteins are nitrogen-containing organic polymers built from amino acids linked by peptide bonds. The sequence of amino acids determines how the chain folds into specific shapes, which lets proteins provide structure to tissues and cells—think of collagen giving strength to connective tissue and keratin in hair and nails. This structural role is a core feature, though proteins also serve many other functions like enzymes and signaling molecules. The other options mix up different biomolecule types: simple sugars are carbohydrates used mainly for energy, lipids store energy and form membranes, and inorganic salts are minerals.