## Core Concept
The core concept being tested here is the structure of immunoglobulins, also known as antibodies, which are crucial components of the immune system. Immunoglobulins are Y-shaped proteins produced by B cells that recognize and bind to specific antigens. Understanding their structure is essential for grasping how they function.

## Why the Correct Answer is Right
The correct answer describes the immunoglobulin structure as consisting of **two heavy chains** and **two light chains** that are linked by **disulfide bonds**, forming a Y-shaped molecule. This structure allows for the specific recognition of antigens through the **variable regions** at the tips of the Y, while the **constant regions** determine the mechanism used to eliminate the antigen. This structure is critical for the antibody's ability to bind to antigens and facilitate their removal from the body.

## Why Each Wrong Option is Incorrect
- **Option A:** This option might suggest an incorrect number of chains or an incorrect description of the immunoglobulin structure, which does not accurately represent the known composition of antibodies.
- **Option B:** Similarly, this option could propose a structure that does not align with the established Y-shaped model composed of heavy and light chains.
- **Option C:** This might describe a partially correct aspect but lacks the complete and accurate description provided by the correct answer.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **variable regions** of the heavy and light chains are responsible for the **antigen specificity** of the antibody, while the **constant regions** of the heavy chains determine the **effector functions**, such as complement activation and binding to Fc receptors on immune cells.

## Correct Answer Line
**Correct Answer: D. Two heavy chains and two light chains linked by disulfide bonds.**