**Core Concept**
During prolonged starvation, the body undergoes adaptive changes to maintain vital organ functions, particularly the brain. The brain initially utilizes glucose as its primary energy source. However, as starvation persists, the body begins to break down alternative energy sources to support cerebral function.

**Why the Correct Answer is Right**
After several weeks of starvation, the brain adapts to utilize ketone bodies as its primary fuel source. This metabolic shift is mediated by the breakdown of fatty acids to produce ketone bodies in the liver. The brain, which is normally dependent on glucose for energy, develops the ability to use ketone bodies as an alternative energy source. This adaptation is crucial for maintaining cognitive function during prolonged periods of caloric deprivation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glucose is the primary fuel source for the brain under normal conditions, but not during prolonged starvation.
* **Option B:** Free fatty acids are not directly used by the brain as an energy source; instead, they are broken down to produce ketone bodies.
* **Option D:** Although the liver plays a critical role in ketone body production, it is not the primary fuel source for the brain during starvation.

**Clinical Pearl / High-Yield Fact**
The brain's ability to adapt to using ketone bodies as an energy source during starvation is a critical mechanism for maintaining cognitive function in patients with intractable epilepsy, type 1 diabetes, or other conditions requiring prolonged caloric restriction.

**Correct Answer: C. Ketone bodies.**