**Core Concept**
Gel filtration chromatography, also known as size-exclusion chromatography, is a type of chromatography that separates molecules based on their size and shape. This technique uses a stationary phase of porous beads, which allows smaller molecules to penetrate deeper into the pores and elute later than larger molecules.

**Why the Correct Answer is Right**
The separation mechanism of gel filtration chromatography is based on the principle that smaller molecules can diffuse into the pores of the stationary phase, while larger molecules are excluded from the pores and elute earlier. The size of the molecules determines how far they penetrate into the pores, with smaller molecules penetrating deeper and eluting later. This technique is useful for separating a wide range of protein sizes, from small peptides to large aggregates.

**Why Each Wrong Option is Incorrect**
**Option A:** Charge-based separation is not the primary mechanism of gel filtration chromatography. While some charge interactions may occur, this is not the primary driving force behind the separation.
**Option B:** Gel filtration chromatography does not separate proteins based on their hydrophobicity. Hydrophobic interaction chromatography (HIC) is a different technique that separates proteins based on their hydrophobic properties.
**Option C:** Gel filtration chromatography is not based on ion exchange. Ion exchange chromatography (IEC) is a different technique that separates proteins based on their charge.

**Clinical Pearl / High-Yield Fact**
Gel filtration chromatography is often used to separate proteins based on their molecular weight, which is a critical factor in understanding protein function and behavior in biological systems.

**Correct Answer: C. Size and shape.**