Step 1: What the Biuret test checks.
This test looks for peptide bonds in a sample. When two or more peptide bonds are present, they grab $Cu^{2+}$ ions in a basic medium and the colour turns violet.
Step 2: Count the peptide bonds in each choice.
A protein is a long chain of amino acids, so it has many peptide bonds. The same is true for peptides and polypeptides. All three pass the test easily.
Step 3: Look at alanine.
Alanine is just one amino acid on its own. A single amino acid has no peptide bond linking it to a neighbour. With no peptide bond, there is nothing for the copper ion to bind, so no violet colour appears.
Step 4: Pick the odd one out.
The only choice without peptide bonds is alanine, so the Biuret test stays negative for it.
\[ \boxed{\text{Alanine}} \]