**Core Concept**
T-cells are a type of lymphocyte that plays a central role in cell-mediated immunity. They are identified by their unique surface receptors and markers, which enable them to recognize and interact with specific antigens.

**Why the Correct Answer is Right**
T-cells are identified by the presence of the T-cell receptor (TCR) on their surface. The TCR is a heterodimeric complex composed of alpha and beta chains (in the case of alpha-beta T-cells) or gamma and delta chains (in the case of gamma-delta T-cells). The TCR recognizes and binds to specific peptide-MHC complexes on the surface of antigen-presenting cells, allowing T-cells to become activated and initiate an immune response. The TCR is a key marker that distinguishes T-cells from other types of lymphocytes.

**Why Each Wrong Option is Incorrect**
**Option A:** CD4 is a marker found on T-helper cells, which is a subset of T-cells, but it is not the primary marker that identifies T-cells as a whole.
**Option B:** CD8 is a marker found on cytotoxic T-cells, which is another subset of T-cells, but it is not the primary marker that identifies T-cells as a whole.
**Option C:** CD19 is a marker found on B-cells, not T-cells.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that T-cells can be broadly categorized into two main subsets: alpha-beta T-cells and gamma-delta T-cells. Alpha-beta T-cells are further divided into T-helper cells (CD4+), cytotoxic T-cells (CD8+), and regulatory T-cells (Tregs).

**Correct Answer: B. CD8. CD8 is a marker found on cytotoxic T-cells, which is a subset of T-cells.**