Biuret Test and its purpose:
The Biuret test is a chemical test used to detect the presence of proteins or polypeptides in a given sample. It is based on the reaction between copper ions and peptide bonds in proteins, resulting in a color change. The test is named after the compound biuret, which contains the necessary functional groups for this reaction to occur.

Understanding the Biuret Test:
The test involves adding a few drops of a Biuret reagent, typically a solution of copper sulfate and sodium hydroxide, to the sample being tested. If proteins or polypeptides are present, a violet or pink color will develop. This color change occurs due to the formation of a coordination complex between copper ions and the peptide bonds in the proteins.

Explanation of the given options:
a) Proteins: Proteins are composed of amino acids, which are linked together by peptide bonds. Therefore, proteins are capable of giving a positive Biuret test.

b) Urea: Urea is a compound that contains a carbonyl group and an amine group, both of which are involved in the formation of peptide bonds. Thus, urea can also give a positive Biuret test.

c) Polypeptide: Polypeptides are chains of amino acids connected by peptide bonds, just like proteins. Hence, polypeptides can also produce a positive Biuret test.

d) Carbohydrates: Carbohydrates, on the other hand, do not contain peptide bonds. They are composed of simple sugars (monosaccharides) or their derivatives, such as disaccharides and polysaccharides. As a result, carbohydrates do not react with the Biuret reagent and do not give a positive test.

Therefore, the correct answer is option 'D' - Carbohydrates. Unlike proteins, urea, and polypeptides, carbohydrates lack the peptide bonds necessary for the formation of the coordination complex with copper ions in the Biuret test. Hence, they do not give a positive result in this test.