**Core Concept:** Protein separation techniques aim to isolate specific proteins based on their molecular characteristics such as size, charge, and structure. The correct answer refers to a method that relies on the difference in size between proteins to separate them.

**Why the Correct Answer is Right:** Gel filtration chromatography (C), also known as size exclusion chromatography, is a laboratory technique used for separating proteins based on their size. In this method, proteins are passed through a gel matrix, and larger proteins will take longer to pass through the gel pores compared to smaller proteins. As a result, the proteins will elute (be released from the column) in a size-dependent manner, with larger proteins eluting first and smaller proteins eluting later. This technique is valuable for studying the molecular weight and structure of proteins.

**Why Each Wrong Option is Incorrect:**

A. Isoelectric focusing (I) is a technique that separates proteins based on their isoelectric points (pI), which are the pH values at which a protein has no net charge. This method is unrelated to protein size.

B. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) is a technique that separates proteins based on their molecular weight and charge, not size.

C. Gel filtration chromatography (C) is correct as mentioned above, separating proteins based on size.

D. Capillary electrophoresis (CE) is a technique that separates proteins based on their size, charge, and shape, but it is not based solely on protein size.

**Clinical Pearl:** Understanding the different protein separation techniques is crucial for biochemists, biologists, and clinical researchers studying protein composition, structure, and function. By choosing the correct method, one can obtain accurate information about proteins' properties, such as molecular weight, charge, and shape. This knowledge is essential for biomedical research, drug development, and clinical diagnostics.