**Core Concept**
NK (Natural Killer) cells are a type of lymphocyte that plays a crucial role in the innate immune system. They are capable of recognizing and killing virally infected cells and tumor cells without prior antigen exposure. The presence of certain surface receptors and markers is critical for NK cell function and identification.

**Why the Correct Answer is Right**
NK cells express the CD56 marker, which is a neural cell adhesion molecule (NCAM) that helps in cell-to-cell adhesion and recognition. CD56 is often used as a marker for NK cells and some subsets of T cells. The presence of CD56 on the surface of NK cells is essential for their interaction with other immune cells and their ability to recognize and target infected or malignant cells.

**Why Each Wrong Option is Incorrect**
**Option A:** CD3 is a marker typically found on T cells, not NK cells. While some NK cells may express low levels of CD3, it is not a characteristic feature of NK cells.

**Option B:** CD19 is a marker found on B cells, not NK cells. CD19 is involved in B cell activation and is often used as a marker for B cell development and malignancies.

**Option C:** CD14 is a marker found on monocytes and macrophages, not NK cells. CD14 is involved in the recognition of bacterial components and is often used as a marker for monocytic lineage cells.

**Clinical Pearl / High-Yield Fact**
NK cells play a critical role in the innate immune response, particularly in the recognition and killing of virally infected cells and tumor cells. The presence of CD56 on NK cells is essential for their function and identification, making it a key marker for NK cell biology.

**Correct Answer: C. CD14**