**Core Concept**
The configuration of sugars, particularly the distinction between L and D forms, is crucial in understanding the stereochemistry of carbohydrates. This concept is fundamental in biochemistry, as it helps explain the behavior and interactions of sugars in biological systems.

**Why the Correct Answer is Right**
The L and D forms of sugars are determined by their relation to glyceraldehyde, a three-carbon sugar that serves as the reference point for sugar configuration. In the Fischer projection, glyceraldehyde is oriented with the aldehyde group at the top, and the hydroxyl group on the right side is considered positive (dextrorotatory). This configuration is used as a reference to classify other sugars as either L or D, depending on their configuration relative to glyceraldehyde.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fructose is a six-carbon sugar that exists primarily in the D form, but it is not used as a reference point for determining the L or D configuration of other sugars.
* **Option B:** Glycogen is a polysaccharide composed of glucose units and is not directly related to the configuration of sugars.
* **Option D:** Glucose is a six-carbon sugar that exists primarily in the D form, but it is not used as a reference point for determining the L or D configuration of other sugars.

**Clinical Pearl / High-Yield Fact**
The Fischer projection is a two-dimensional representation of sugar molecules, which helps to visualize their stereochemistry and configuration. Understanding this concept is crucial in biochemistry, as it helps explain the behavior and interactions of sugars in biological systems.

**✓ Correct Answer: C. Glyceraldehyde**