Biuret test is used for the detection of:

Options

A. sugar
B. proteins
C. fats
D. starch

Explanation

### Step-by-Step Solution:

1. **Understanding the Biuret Test**: The Biuret test is a chemical test used to detect the presence of proteins in a sample. It is based on the formation of a color change when proteins are present.

2. **Components of the Test**: The test involves an alkaline solution (usually sodium hydroxide) and a few drops of 1% copper sulfate (CuSO4) solution.

3. **Reaction Mechanism**: When the alkaline solution of the protein or polypeptide is treated with CuSO4, a reaction occurs. The Cu²⁺ ions from the copper sulfate interact with the peptide bonds present in the protein.

4. **Color Change Observation**: The presence of proteins is indicated by the formation of a violet color. This color change is due to the formation of a coordination complex between the Cu²⁺ ions and the nitrogen atoms in the peptide bonds.

5. **Conclusion**: The Biuret test is specifically used for the detection of proteins, as it relies on the presence of peptide bonds (C-O-N-H bonds) found in proteins and polypeptides.

### Final Answer:
The Biuret test is used for the detection of **proteins**.

---