## Core Concept
The question tests knowledge of histological staining techniques, specifically those used to stain cell nuclei. Various stains are used in histology to differentiate cellular components, and understanding their specific applications is crucial.

## Why the Correct Answer is Right
Hematoxylin is a commonly used stain in histology, particularly for staining nuclei. It works by binding to the acidic components of the cell, such as DNA and RNA, which are predominantly found in the nucleus. This affinity for nucleic acids makes hematoxylin an ideal stain for highlighting nuclear structures.

## Why Each Wrong Option is Incorrect
* **Option A:** Eosin is primarily used to stain the cytoplasm and other cellular components, not the nucleus. It is often used in conjunction with hematoxylin in H&E (Hematoxylin and Eosin) staining.
* **Option B:** This option might seem plausible but is not the best choice for nuclear staining.
* **Option D:** This option is not typically associated with nuclear staining.

## Clinical Pearl / High-Yield Fact
A key point to remember is that H&E staining is one of the most commonly used techniques in histopathology. Hematoxylin stains nuclei blue or purple, while eosin stains the cytoplasm pink. This contrasting staining helps in the detailed examination of tissue morphology.

## Correct Answer Line
**Correct Answer: C. Hematoxylin.**