**Core Concept**
The question is testing the student's knowledge of CD markers, specifically the ones associated with cytotoxic T cells. CD markers are proteins on the surface of immune cells that help identify and classify them. In this case, we're looking for the CD marker that is characteristic of cytotoxic T cells.

**Why the Correct Answer is Right**
Cytotoxic T cells, also known as CD8+ T cells, play a crucial role in the immune system by killing infected cells or tumor cells. They express a specific CD marker, CD8, which is a receptor that interacts with MHC class I molecules on the surface of target cells. This interaction triggers the cytotoxic T cell to release cytotoxins that ultimately lead to cell death. The CD8 marker is essential for the cytotoxic function of these T cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CD4 is a marker associated with helper T cells, not cytotoxic T cells.
**Option B:** This option is incorrect because CD19 is a marker associated with B cells, not T cells.
**Option C:** This option is incorrect because CD56 is a marker associated with natural killer cells, not cytotoxic T cells.

**Clinical Pearl / High-Yield Fact**
Remember that CD8+ T cells are the primary cytotoxic T cells in the immune system, and they play a crucial role in controlling viral infections and tumor growth.

**Correct Answer: C. CD56**