**Core Concept**
The B-cell receptor (BCR) is a complex structure on the surface of B lymphocytes that plays a crucial role in the adaptive immune response. It is composed of an antigen-binding component and a signaling component. The antigen-binding component is responsible for recognizing and binding to specific antigens, while the signaling component transduces the signal to the cell.

**Why the Correct Answer is Right**
The antigen-binding component of the B-cell receptor is composed of immunoglobulin molecules. Specifically, it is the heavy chain and light chain variable regions (VH and VL) that form the antigen-binding site. The VH and VL regions are highly diverse and are responsible for recognizing and binding to specific antigens. The correct immunoglobulin that constitutes the antigen-binding component of the B-cell receptor is IgM, which is the first antibody to be produced in response to an antigen.

**Why Each Wrong Option is Incorrect**
* **Option A:** IgG is the most abundant class of antibodies in the blood and plays a crucial role in secondary immune responses, but it is not the primary antigen-binding component of the B-cell receptor.
* **Option B:** IgE is involved in allergic reactions and is not typically associated with the B-cell receptor.
* **Option D:** IgD is present on the surface of mature B cells and is thought to play a role in the recognition of antigens, but it is not the primary antigen-binding component of the B-cell receptor.

**Clinical Pearl / High-Yield Fact**
The B-cell receptor is a key component of the adaptive immune response, and its dysfunction can lead to a range of immunological disorders, including autoimmune diseases and immunodeficiencies. Understanding the structure and function of the B-cell receptor is essential for the diagnosis and treatment of these conditions.

**Correct Answer:** C.