**Core Concept**
The question is testing the knowledge of the specific receptors present on T-cells, a crucial component of the immune system. T-cells, also known as T-lymphocytes, play a central role in cell-mediated immunity and are characterized by the presence of specific surface receptors.

**Why the Correct Answer is Right**
The correct answer, CD4, is a type of receptor present on the surface of T-cells, specifically on helper T-cells. CD4 receptors are a subtype of adhesion molecules that facilitate the interaction between T-cells and antigen-presenting cells (APCs). The CD4 receptor is essential for the activation of T-cells, as it recognizes and binds to the major histocompatibility complex (MHC) class II molecules on APCs. This binding event triggers a series of signaling pathways that lead to T-cell activation and proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** IgA is a type of antibody that plays a key role in mucosal immunity, but it is not a receptor present on T-cells. IgA is primarily produced by B-cells and is involved in the neutralization of pathogens at mucosal surfaces.
**Option B:** IgG is another type of antibody that is crucial for humoral immunity, but like IgA, it is not a receptor present on T-cells. IgG is primarily produced by B-cells and is involved in the neutralization of pathogens in the bloodstream.
**Option C:** Prostaglandins are a group of lipid compounds that are involved in various physiological processes, including inflammation and immune response. However, they are not receptors present on T-cells.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the interaction between CD4 receptors on T-cells and MHC class II molecules on APCs is a critical step in the activation of T-cells and the initiation of cell-mediated immunity. This interaction is also the target of HIV, which uses the CD4 receptor as a co-receptor to gain entry into host cells.

**✓ Correct Answer: D. CD4. CD4 receptors are present on the surface of T-cells.**