**Core Concept**
The question tests knowledge of cell membrane structure, focusing on the component with the highest protein content. Cell membranes are composed of **phospholipid bilayers** and embedded proteins, which perform various functions.

**Why the Correct Answer is Right**
The correct answer is related to the **cell membrane's** structure and function, specifically where proteins are most concentrated. Proteins in the membrane can act as **receptors**, **enzymes**, or **transport proteins**, facilitating interactions between the cell and its environment.

**Why Each Wrong Option is Incorrect**
**Option A:** Not specified, but typically, options like this might refer to other cellular structures or organelles.
**Option B:** Similarly, without the text, we can infer that incorrect options might include organelles or structures not primarily known for high protein content in the context of membranes.
**Option C:** This could potentially refer to a structure or component not primarily composed of proteins.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **plasma membrane** is indeed rich in proteins, which are crucial for cell signaling, transport, and other vital functions. Understanding membrane structure is essential for grasping many pathological processes.

**Correct Answer:** D. Plasma membrane.