**Core Concept**
The immunoglobulin that mediates Antibody-Dependent Cellular Cytotoxicity (ADCC) is a key component of the immune system, facilitating the destruction of target cells. **ADCC** is a mechanism through which certain immune cells can lyse target cells that have been opsonized (or marked) by antibodies. This process involves the binding of antibodies to specific antigens on the surface of target cells.

**Why the Correct Answer is Right**
The correct answer is related to the **IgG** subclass of immunoglobulins, which are known to effectively mediate **ADCC**. This is because **IgG** antibodies can bind to the **Fc receptors** on the surface of immune effector cells, such as natural killer (NK) cells and macrophages, thereby facilitating the recognition and destruction of target cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specifically relate to the primary mediator of ADCC.
**Option B:** Incorrect as it is not the primary class of immunoglobulins involved in ADCC.
**Option C:** Incorrect because, although important in immune responses, it is not the primary mediator of ADCC.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **IgG** is the most common antibody isotype found in blood circulation and plays a crucial role in the secondary immune response, including the mediation of **ADCC**.

**Correct Answer:** D. IgG1