**Core Concept**
The Feulgen reaction is a histological staining technique used to detect DNA in cell nuclei. It is based on the **Schiff reaction**, which involves the hydrolysis of DNA by hydrochloric acid to form aldehyde groups. These aldehyde groups then react with the Schiff reagent to produce a purple stain.

**Why the Correct Answer is Right**
The Feulgen reaction is specific for DNA, making it a useful tool for identifying cells that are in the process of dividing or for detecting DNA in tissue samples. The reaction involves the hydrolysis of DNA by **hydrochloric acid**, followed by the reaction with the **Schiff reagent**. This results in a purple stain that is proportional to the amount of DNA present in the cell nucleus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Feulgen reaction is not used to test for proteins.
**Option B:** This option is incorrect because the Feulgen reaction is not used to test for lipids.
**Option C:** This option is incorrect because the Feulgen reaction is not used to test for carbohydrates.

**Clinical Pearl / High-Yield Fact**
The Feulgen reaction is a valuable tool in **cytogenetics** and **histopathology**, allowing for the visualization of DNA in cell nuclei. This can be useful in diagnosing certain types of cancer or genetic disorders.

**Correct Answer:** D. DNA