## Core Concept
The fluidity of a cell membrane is determined by its composition, particularly the types of lipids and the presence of cholesterol. Cell membrane fluidity is crucial for various cellular functions, including transport, signaling, and cell division. Factors that alter membrane fluidity can impact these functions.

## Why the Correct Answer is Right
Cholesterol is known to decrease the fluidity of cell membranes. It does so by filling the spaces between phospholipid molecules, thereby reducing the movement of the phospholipid tails. This effect is most pronounced at temperatures that are near the melting point of the membrane lipids. At higher temperatures, cholesterol can also limit the movement of phospholipid molecules, thus reducing fluidity.

## Why Each Wrong Option is Incorrect
* **Option A:** Increasing the length of fatty acid chains in phospholipids generally decreases membrane fluidity because longer chains have more opportunities to interact with each other, making the membrane less fluid. However, this option is not specified, so we cannot confirm its correctness or incorrectness based on the information given.
* **Option B:** Decreasing the saturation of fatty acid chains (increasing unsaturation) increases membrane fluidity. Unsaturated fatty acids have kinks in their tails due to double bonds, which prevent them from packing tightly together, thus increasing fluidity.
* **Option C:** Increasing the temperature generally increases membrane fluidity by providing more energy for the phospholipid molecules to move.

## Clinical Pearl / High-Yield Fact
A key point to remember is that cholesterol plays a significant role in maintaining optimal membrane fluidity. Its levels are tightly regulated within cells, and alterations in cholesterol levels can affect membrane function. For example, in cold climates, some animals increase the unsaturated fatty acid content in their membranes to maintain fluidity, while cholesterol's role is more about maintaining a balance across varying temperatures.

## Correct Answer: C. Cholesterol.