**Core Concept**
Sterols are a subgroup of steroids that are characterized by the presence of an hydroxyl group at the 3-position of the steroid nucleus. They play a crucial role in various biological processes, including cell membrane structure and function.

**Why the Correct Answer is Right**
The correct answer involves understanding the biosynthesis and distribution of sterols. Sterols are synthesized in the cytoplasmic endoplasmic reticulum and subsequently transported to the cell membrane, where they are incorporated into the phospholipid bilayer. The most common sterol is cholesterol, which is essential for maintaining the fluidity and stability of cell membranes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because sterols are not typically found in the mitochondria. While mitochondria do contain some sterol-related enzymes, they are not a primary site for sterol accumulation.
**Option B:** This option is incorrect because phospholipids, not sterols, are the primary components of cell membranes. While sterols can interact with phospholipids, they are not the primary structural element of cell membranes.
**Option C:** This option is incorrect because while sterols are found in the plasma membrane, they are not exclusively localized to this compartment. Sterols can also be found in other cellular compartments, such as the endoplasmic reticulum and Golgi apparatus.

**Clinical Pearl / High-Yield Fact**
Cholesterol is an essential component of cell membranes, and its deficiency can lead to membrane instability and dysfunction. This is particularly relevant in the context of atherosclerosis, where cholesterol accumulation in arterial walls contributes to plaque formation and cardiovascular disease.

**Correct Answer:** C. Cell membranes.