**Core Concept**
Immunoglobulins, also known as antibodies, play a crucial role in the immune system. They are produced by B cells and can act as receptors on their surface, helping to recognize and bind to specific antigens.

**Why the Correct Answer is Right**
The correct answer is **IgM**, as it acts as the primary receptor on the surface of mature B cells. This receptor is responsible for recognizing and binding to antigens, which triggers the activation of B cells and the initiation of an immune response. The membrane-bound form of IgM is composed of five immunoglobulin units, each containing a light and a heavy chain, which allows it to bind to antigens with high avidity.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is the most abundant class of immunoglobulins in the blood, but it does not act as a receptor on B cells. Instead, it plays a crucial role in the secondary immune response and provides long-term protection against infections.

**Option B:** IgE is involved in the allergic response and is produced in response to allergens. It does not act as a receptor on B cells and is instead responsible for triggering mast cell degranulation and the release of histamine.

**Option C:** IgA is primarily found in mucosal secretions, such as saliva, tears, and breast milk, where it provides protection against infections. It does not act as a receptor on B cells.

**Clinical Pearl / High-Yield Fact**
Remember that the receptor on B cells is IgM, and it plays a crucial role in the initiation of an immune response. This is an important concept to remember when considering the mechanisms of immune system activation and the roles of different immunoglobulins.

**Correct Answer:** C. IgM.