**Core Concept**
The Feulgen reaction is a cytochemical staining technique used for the detection of DNA in tissue sections. It is a specific method for visualizing the presence of nucleic acids, particularly DNA, in cells.

**Why the Correct Answer is Right**
The Feulgen reaction involves the hydrolysis of DNA with acid, followed by a Schiff reagent that stains the aldehydes produced from the DNA hydrolysis. This results in a purple coloration that is proportional to the amount of DNA present in the cell. The reaction is specific for DNA because the Schiff reagent reacts with the aldehydes produced from the hydrolysis of the DNA sugar-phosphate backbone. This makes it an ideal method for quantifying DNA content in cells, which is useful for assessing cell growth, differentiation, and viability.

**Why Each Wrong Option is Incorrect**
**Option A:** The Alcian blue stain is used to detect glycosaminoglycans, not DNA.
**Option B:** The PAS (Periodic Acid-Schiff) stain is used to detect carbohydrates, not DNA.
**Option C:** The Giemsa stain is used to stain chromosomes and distinguish between different cell types, but it is not specific for DNA detection.

**Clinical Pearl / High-Yield Fact**
The Feulgen reaction is a classic example of a cytochemical staining technique that has been used for decades to study cell biology and pathology. It is still widely used today in research and diagnostic settings to study the structure and function of cells.

**Correct Answer: D. DNA**