**Core Concept**
Ion exchange chromatography is a type of chromatography that separates molecules based on their ionic properties. It relies on the interaction between charged molecules and charged stationary phases, allowing for the separation of ions with different charges.

**Why the Correct Answer is Right**
Ion exchange chromatography exploits the electrostatic attraction between oppositely charged species, typically between positively charged molecules and negatively charged stationary phases, or vice versa. This interaction enables the separation of ions with different charges, making it a powerful tool for purifying and analyzing biological molecules. The process involves the exchange of ions between the stationary phase and the mobile phase, allowing for the selective binding and elution of charged molecules.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because ion exchange chromatography is not based on size exclusion or hydrophobic interactions, which are the principles behind size exclusion chromatography and hydrophobic interaction chromatography, respectively.
**Option B:** This option is incorrect because ion exchange chromatography does not rely on the difference in solubility between two compounds, which is the principle behind partition chromatography.
**Option C:** This option is incorrect because ion exchange chromatography is not based on the difference in density between two compounds, which is the principle behind density gradient centrifugation.

**Clinical Pearl / High-Yield Fact**
Ion exchange chromatography is widely used in biochemistry and molecular biology to separate and purify proteins, peptides, and other biomolecules. It is a crucial technique in protein purification and analysis, and has numerous applications in fields such as biotechnology, pharmaceuticals, and clinical diagnostics.

**Correct Answer: A. Electrostatic interaction**