**Core Concept:**
The plasma membrane, also known as the cell membrane or the غشاء الخلية (غشاء الخلية in Arabic), is a lipid bilayer that surrounds the cell and plays a crucial role in maintaining the cell's integrity, selectively permitting or preventing the passage of molecules and ions, and interacting with the extracellular environment. It is composed of lipids, proteins, and carbohydrates.

**Why the Correct Answer is Right:**
The main component of the plasma membrane is lipids, specifically phospholipids. Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) parts. These phospholipids form a double layer, with the hydrophilic heads facing the aqueous environments (cytosol and extracellular fluid) and the hydrophobic tails together forming the core of the membrane. This arrangement creates a barrier that allows selective permeability, enabling the cell to maintain its internal environment and interact with the extracellular environment as needed.

**Why Each Wrong Option is Incorrect:**
A. Proteins: While proteins are indeed present in the plasma membrane, they are embedded within or attached to the lipid bilayer, not a separate component.
B. Carbohydrates: While carbohydrates are present in the plasma membrane, they are attached to proteins forming the glycocalyx or glycolipids, not a separate component.
C. Membrane fluidity: While membrane fluidity is essential, it is not an independent component, but rather influenced by the lipid composition and temperature.

**Clinical Pearl:**
Understanding the composition and structure of the plasma membrane is essential in understanding cellular function, transport mechanisms, and drug targeting. The plasma membrane is a dynamic structure that adapts to the cell's needs, such as during cell signaling, immune response, or response to temperature changes.

**Correct Answer:** D (lipids)

**Explanation:**
The lipid bilayer, comprising phospholipids, is the fundamental component of the plasma membrane. It provides structural integrity, selective permeability, and allows for the incorporation of proteins and carbohydrates in the form of glycolipids, glycoproteins, and proteins embedded or attached to the lipid bilayer. This unique composition enables the cell to maintain its internal environment, interact with the extracellular environment, and facilitate various cellular processes, such as transport, signaling, and immune response.