**Core Concept**
The complement system is a group of proteins that play a crucial role in the innate immune response, helping to clear pathogens from the body. Immunoglobulins (antibodies) can activate the complement system by binding to specific receptors, leading to the formation of a membrane attack complex (MAC) that disrupts the pathogen's cell membrane.

**Why the Correct Answer is Right**
The Fc region of immunoglobulins is responsible for interacting with various receptors and proteins, including the complement system. Specifically, the Fc region of IgM and IgG antibodies can bind to the C1q protein, a key component of the classical pathway of complement activation. This binding causes a conformational change in C1q, leading to the activation of the C1r and C1s enzymes, which then cleave and activate C4 and C2, ultimately forming the C3 convertase.

**Why Each Wrong Option is Incorrect**
**Option A:** IgE is primarily involved in allergic reactions and does not play a significant role in complement activation.
**Option B:** IgD is mainly expressed on the surface of mature B cells and does not have a significant function in complement fixation.
**Option C:** IgA is primarily involved in mucosal immunity and does not have a significant function in complement fixation through the Fc region.

**Clinical Pearl / High-Yield Fact**
The classical pathway of complement activation is more efficient at activating the complement system compared to the alternative pathway, which is more important in the clearance of apoptotic cells and other non-pathogenic particles.

**Correct Answer:** D. IgM.