**Core Concept**
The biuret test is a chemical assay used to detect the presence of proteins in a sample. It relies on the reaction between peptides and copper ions in an alkaline environment, resulting in a purple-colored complex.

**Why the Correct Answer is Right**
The biuret test is based on the principle of peptide bond formation between peptides and copper ions, which leads to the formation of a purple-colored complex. This reaction is specific to proteins, making it a reliable method for detecting their presence. The reaction involves the reduction of copper ions to copper(I) ions, which then form a stable complex with the peptides. This complex is responsible for the characteristic purple color observed in the biuret test.

**Why Each Wrong Option is Incorrect**
* **Option A:** The biuret test is not used to detect nucleic acids, which can be detected using other methods such as the diphenylamine reaction.
* **Option B:** The biuret test is not specific to carbohydrates, which can be detected using other methods such as the Benedict's test.
* **Option C:** The biuret test is not used to detect lipids, which can be detected using other methods such as the Sudan III stain.

**Clinical Pearl / High-Yield Fact**
The biuret test is a simple and reliable method for detecting proteins in a sample, and it is widely used in various clinical and research settings.

**Correct Answer: D. Proteins.**