**Core Concept**
The cell membrane, also known as the plasma membrane, is a lipid bilayer composed of various components. The main function of the cell membrane is to regulate the movement of substances in and out of the cell. The composition of the cell membrane is crucial for maintaining cellular homeostasis.

**Why the Correct Answer is Right**
The correct answer is cholesterol, which is the most abundant sterol in the cell membrane. Cholesterol plays a vital role in maintaining the fluidity and structure of the cell membrane. It helps to prevent the cell membrane from becoming too rigid or too fluid, allowing for proper cell signaling and transport of molecules. Cholesterol is embedded between the phospholipid molecules, forming a complex structure that is essential for cell membrane function.

**Why Each Wrong Option is Incorrect**
**Option A:** Phospholipids are the primary components of the cell membrane, but they are not the most abundant component. Phospholipids are the building blocks of the cell membrane, forming a bilayer structure that separates the cell from its environment.
**Option B:** Glycoproteins and glycolipids are important components of the cell membrane, but they are not the most abundant. These molecules are involved in cell signaling, cell adhesion, and other cellular processes.
**Option C:** Proteins are essential components of the cell membrane, but they are not the most abundant. Proteins are involved in various cellular processes, including transport, signaling, and cell adhesion.

**Clinical Pearl / High-Yield Fact**
Cholesterol is essential for maintaining the fluidity of the cell membrane, and alterations in cholesterol levels can lead to changes in cell membrane function. For example, high cholesterol levels can lead to atherosclerosis, a condition characterized by the buildup of plaque in the arteries.

**Correct Answer: C. Cholesterol**