**Core Concept**
Memory T cells, a subset of T lymphocytes, play a crucial role in long-term immune surveillance and rapid response to pathogens. They are characterized by their ability to proliferate and differentiate into effector cells upon antigen re-encounter.

**Why the Correct Answer is Right**
Memory T cells can be identified by the expression of the **CD45RO** marker, which is a splice variant of the CD45 protein. This marker is typically expressed on memory T cells but not on naive T cells. CD45RO is involved in signaling through the T cell receptor (TCR) complex and is essential for T cell activation and proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** CD4 is a marker expressed on helper T cells, which is not specific for memory T cells.
**Option B:** CD8 is a marker expressed on cytotoxic T cells, which is not specific for memory T cells.
**Option C:** CD25 is a marker expressed on activated T cells, which is not specific for memory T cells.

**Clinical Pearl / High-Yield Fact**
CD45RO is often used as a marker for memory T cells in immunohistochemistry and flow cytometry. This marker can help distinguish between memory and naive T cells, which is essential for understanding immune responses and immune disorders.

**Correct Answer:** C. CD25