**Core Concept**
The plasmalemma, also known as the plasma membrane, is a semi-permeable lipid bilayer that surrounds the cell and regulates the movement of substances in and out. Cholesterol plays a crucial role in maintaining the structure and fluidity of the plasmalemma.

**Why the Correct Answer is Right**
Cholesterol is embedded within the lipid bilayer of the plasmalemma and helps to reduce the fluidity of the membrane by filling the gaps between phospholipid molecules. This is essential for maintaining the stability of the membrane and preventing it from becoming too fluid, which could lead to the loss of cellular function. Additionally, cholesterol is involved in the formation of lipid rafts, which are specialized membrane domains that facilitate cell signaling and other cellular processes.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cholesterol is not primarily involved in the regulation of ion channels in the plasmalemma. While it does play a role in maintaining the structure of the membrane, its primary function is not related to ion channel regulation.
* **Option B:** Cholesterol is not primarily involved in the synthesis of phospholipids. While it is a component of the plasmalemma, its primary function is not related to phospholipid synthesis.
* **Option D:** Cholesterol is not primarily involved in the regulation of water transport across the plasmalemma. While it does play a role in maintaining the structure of the membrane, its primary function is not related to water transport.

**Clinical Pearl / High-Yield Fact**
Cholesterol is essential for maintaining the integrity of the plasmalemma, and alterations in its levels can have significant effects on cellular function. For example, elevated cholesterol levels can lead to the formation of foam cells, which are a hallmark of atherosclerosis.

**Correct Answer: C. Maintaining the structure and fluidity of the plasmalemma.**