**Core Concept**
The in-situ DNA nick end labeling technique, also known as TUNEL (TdT-mediated dUTP nick end labeling), is a method used to detect apoptotic cells in tissue sections. It involves the labeling of DNA strand breaks with nucleotides that are incorporated by the enzyme terminal deoxynucleotidyl transferase (TdT). This technique is essential for identifying cells undergoing programmed cell death.

**Why the Correct Answer is Right**
The TUNEL assay is specifically designed to detect DNA fragmentation, which is a hallmark of apoptosis. The process involves the incubation of tissue sections with dUTP nucleotides that are labeled with a fluorescent marker. TdT then adds nucleotides to the free 3'-hydroxyl ends of DNA strands, resulting in the formation of a labeled DNA strand. This labeled DNA can be visualized using fluorescence microscopy, allowing researchers to identify cells undergoing apoptosis. The TUNEL assay is widely used in various fields, including cancer research, neurology, and immunology.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely referring to a different staining technique, such as hematoxylin and eosin (H&E) staining, which is used for general tissue morphology.
**Option B:** This option might be referring to a technique for detecting cell proliferation, such as Ki-67 immunostaining, which is not directly related to apoptosis.
**Option C:** This option could be related to a technique for detecting protein expression, such as Western blotting, which is not relevant to the detection of apoptotic cells.

**Clinical Pearl / High-Yield Fact**
The TUNEL assay is a crucial tool for researchers to study apoptosis and its role in various diseases, including cancer. It is essential to understand the principles behind this technique to accurately interpret results and make informed conclusions.

**Correct Answer:** C. TUNEL assay is used to detect apoptotic cells.