**Core Concept**
In the fasting state, the body's metabolic state shifts from utilizing glucose as the primary fuel source to using alternative energy sources. This adaptation is crucial for maintaining energy homeostasis during periods of food deprivation.

**Why the Correct Answer is Right**
When fasting, the body relies heavily on stored glycogen for initial energy release. However, as glycogen stores deplete, the liver and kidneys produce ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) from fatty acid breakdown. These ketone bodies become the preferred fuel for the brain, heart, and other tissues, reducing dependence on glucose. This metabolic shift is mediated by the hormone glucagon, which increases lipolysis and ketogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose is the primary fuel source in the fed state, not the fasting state.
**Option B:** Adipose tissue is the primary storage site for lipids, but it's not the preferred fuel source during fasting.
**Option C:** Amino acids can be used as fuel, but they're not the preferred source during prolonged fasting.

**Clinical Pearl / High-Yield Fact**
During prolonged fasting, the brain can adapt to use ketone bodies as a primary fuel source, reducing the risk of hypoglycemia and improving cognitive function.

**Correct Answer:** C. Fatty acids.