**Core Concept**
The CD4 marker is a crucial surface glycoprotein expressed on the surface of a subset of immune cells, playing a pivotal role in the immune system's function. CD4 is a key component of the T-cell receptor complex, facilitating the interaction between T-cells and antigen-presenting cells.

**Why the Correct Answer is Right**
CD4 is specifically expressed on the surface of T-helper cells, also known as CD4+ T-cells. These cells are a subset of T-cells that play a vital role in the adaptive immune response by stimulating the activation of B-cells and other immune cells. The CD4 molecule acts as a co-receptor for the T-cell receptor, enhancing the binding of T-cells to major histocompatibility complex (MHC) class II molecules on antigen-presenting cells. This interaction is essential for the activation and proliferation of T-cells.

**Why Each Wrong Option is Incorrect**
**Option A:** CD4 is not a marker for stem cells, which are the earliest cells in the hematopoietic lineage. Stem cells give rise to all blood cell types, but CD4 is specific to T-cells.
**Option B:** CD4 is not a marker for all leukocytes, as it is specific to a subset of T-cells and not expressed on all white blood cells.
**Option C:** CD4 is not a marker for B-cells, which are a different type of immune cell that primarily produce antibodies. B-cells express different surface molecules, such as CD19 and CD20.

**Clinical Pearl / High-Yield Fact**
CD4 counts are often used as a surrogate marker for the progression of HIV infection and the effectiveness of antiretroviral therapy. A decline in CD4 count is associated with a weakened immune system and increased susceptibility to opportunistic infections.

**✓ Correct Answer: D. T - cells**