**Core Concept**
Phospholipids are a class of lipids that form a major component of cell membranes, playing a crucial role in maintaining the structural integrity and fluidity of cell membranes. They consist of a phosphate group, glycerol, and two fatty acid chains.

**Why the Correct Answer is Right**
Phospholipids are amphipathic molecules, meaning they have both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions. This property allows them to form lipid bilayers, which are essential for cell membrane structure and function. The phosphate group is hydrophilic, while the fatty acid chains are hydrophobic. This unique structure enables phospholipids to interact with both water and non-polar substances, making them ideal for cell membrane formation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because cholesterol is a type of sterol, not a phospholipid. While cholesterol is also found in cell membranes, it has a different chemical structure and function.
* **Option B:** This option is incorrect because sphingomyelin is a type of sphingolipid, not a phospholipid. Sphingomyelin is also found in cell membranes, but it has a distinct chemical structure and function.
* **Option C:** This option is incorrect because glycerol is a component of phospholipids, but it is not a phospholipid itself. Glycerol is a simple sugar that forms the backbone of phospholipids.

**Clinical Pearl / High-Yield Fact**
Phospholipids are essential for maintaining cell membrane fluidity and structure. Changes in phospholipid composition can lead to alterations in cell membrane function, contributing to various diseases, such as atherosclerosis and certain types of cancer.

**Correct Answer: D. Glycogen**