**Core Concept**
The question is testing the understanding of protein separation techniques based on molecular size. This concept is crucial in biochemistry and molecular biology, where the separation and purification of proteins are essential for various applications, including research, diagnostics, and therapeutics.

**Why the Correct Answer is Right**
The technique that depends on the molecular size of the protein is gel filtration or size exclusion chromatography. This method exploits the principle that smaller molecules will pass through the pores of the gel matrix more easily than larger molecules, resulting in a separation based on molecular size. The gel matrix, typically made of dextran or agarose, has pores of specific sizes that allow molecules to pass through, and the separation is achieved based on the size of the molecules relative to the pore size. The smaller molecules will elute first, followed by the larger molecules.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ion exchange chromatography is a technique that separates proteins based on their charge, not molecular size. It involves the interaction between the protein and the ionized groups on the chromatographic matrix.
* **Option B:** Affinity chromatography separates proteins based on their specific binding to a ligand attached to the chromatographic matrix, not molecular size.
* **Option C:** High-performance liquid chromatography (HPLC) is a broad category of techniques that can separate proteins based on various properties, including size, charge, and hydrophobicity. However, it is not specific to molecular size.

**Clinical Pearl / High-Yield Fact**
When working with protein separation techniques, it's essential to understand the principles behind each method to choose the most suitable technique for a specific application. Gel filtration is particularly useful for separating proteins of varying sizes, such as enzymes and antibodies.

**Correct Answer:** C.