What does the Biuret test detect?

The Biuret test is used to detect the presence of peptide bonds $(-CO-NH-)$ in a compound.

Compounds containing two or more peptide linkages give a violet/purple color with alkaline copper sulfate solution.

Check each option:

A: Carbohydrates

Carbohydrates do not contain peptide bonds. Therefore, they do not give the Biuret test.

B: Polypeptides

Polypeptides contain many peptide bonds, so they give the Biuret test.

C: Urea

Although the name “Biuret test” is historically related to the compound biuret formed from heating urea, urea itself does not contain peptide bonds and hence does not give the Biuret test.

D: Proteins

Proteins contain many peptide bonds, so they give the Biuret test.

Important note

Strictly chemically, both carbohydrates and urea do not give the Biuret test.

So if interpreted rigorously as a single-correct MCQ, the question is ambiguous because there are two options that do not give the test.

Most likely exam intent

In standard biomolecules context, the expected contrast is:

• proteins/polypeptides: give Biuret test
• carbohydrates: do not give Biuret test

Hence the intended answer is usually A.

Final answer

$\boxed{A\text{ : carbohydrates}}$