**Core Concept**
T cells, also known as T lymphocytes, play a central role in cell-mediated immunity. They recognize and respond to antigens presented by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells (APCs). This recognition is mediated by specific receptors on the surface of T cells.

**Why the Correct Answer is Right**
The correct answer is CD4, which is a type of receptor present on the surface of helper T cells (Th cells). CD4 receptors are crucial for the activation of T cells, as they bind to MHC class II molecules on APCs. This binding facilitates the interaction between T cells and APCs, enabling the presentation of antigens to T cells. The CD4 receptor is a key component of the T cell receptor (TCR) complex, which recognizes and responds to specific antigens.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is a type of antibody, not a receptor on T cells. Antibodies are produced by B cells and play a key role in humoral immunity.

**Option B:** IgD is another type of antibody, also produced by B cells. Like IgG, it is not a receptor on T cells.

**Option D:** Prostaglandins are a group of lipid compounds that have various physiological effects, including inflammation and pain. They are not receptors on T cells.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that CD4 receptors are selectively expressed on helper T cells, whereas cytotoxic T cells express CD8 receptors. This distinction is critical in understanding T cell function and the pathogenesis of diseases such as HIV/AIDS, which targets CD4+ T cells.

**✓ Correct Answer: C. CD4 CD4 receptors on T cells.**