**Core Concept**
In humans, carbohydrates are primarily stored as a complex molecule that provides energy to cells. This storage form plays a crucial role in maintaining blood sugar levels and supplying energy to tissues.

**Why the Correct Answer is Right**
The correct answer is glycogen. Glycogen is a polysaccharide stored in the liver and muscles, serving as a readily available source of glucose for the body. It is synthesized from glucose through the action of the enzyme glycogen synthase, which adds glucose molecules to a growing glycogen chain. Glycogen is broken down into glucose through the action of glycogen phosphorylase, releasing glucose-6-phosphate into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** Fat is incorrect because it is a different type of macromolecule that stores energy in the form of triglycerides, not carbohydrates.
**Option B:** Protein is incorrect because it is a polypeptide chain of amino acids and is not a storage form of carbohydrates.
**Option C:** Cellulose is incorrect because it is a polysaccharide found in plant cell walls, not in human tissues.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that glycogen storage diseases, such as glycogen storage disease type I (GSD I), can lead to severe hypoglycemia due to impaired glycogen breakdown.

**Correct Answer:** C. Glycogen.