Biuret test detects proteins from amino acid monomers with carboxyl groups

Steps:

• Recall biological monomers with carboxyl groups: amino acids (-COOH in proteins) and fatty acids (in lipids).
• Match tests to biomolecules: biuret detects proteins via peptide bonds; Benedict's detects reducing sugars (no carboxyl in standard monomers); ethanol emulsion detects lipids.
• Evaluate choices: only biuret directly identifies proteins with relevant monomers; combinations including Benedict's add irrelevant sugar test.
• Select single test option that fits without extraneous ones.

Why the correct option is correct:

• Biuret test identifies proteins, polymers of amino acids that each contain a carboxyl group (-COOH), confirming the molecule's monomeric composition.

Why the others are wrong:

• A: Benedict's tests reducing sugars like glucose, whose monomers lack carboxyl groups.
• B: Includes Benedict's (for sugars without carboxyl monomers) alongside ethanol (for lipids).
• D: Includes Benedict's (irrelevant for carboxyl-containing monomers) alongside biuret.

Final answer: C