**Core Concept**
The structure of an immunoglobulin molecule is crucial for its function in the immune system. An immunoglobulin, or antibody, is a Y-shaped protein composed of multiple polypeptide chains. These chains determine the antibody's class and specificity.

**Why the Correct Answer is Right**
A single immunoglobulin molecule typically contains two heavy chains and two light chains, which are linked by disulfide bonds. The heavy chains determine the class of the antibody (e.g., IgG, IgM), while the light chains contribute to the antibody's specificity for antigens. This basic structure is common to all antibodies, regardless of their class or function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the basic structure of an immunoglobulin molecule.
**Option B:** Similarly, this option is incorrect as it misrepresents the composition of an immunoglobulin molecule.
**Option D:** This option is also incorrect because it does not match the known structure of an immunoglobulin molecule.

**Clinical Pearl / High-Yield Fact**
The structure of an immunoglobulin molecule is key to understanding how antibodies function in the immune response. Remembering that each molecule contains two heavy and two light chains is crucial for understanding antibody diversity and function.

**Correct Answer:** D. 2 heavy chains and 2 light chains.