**Core Concept**
SDS-PAGE is a laboratory technique used for protein separation and analysis. It relies on the principle of charge-to-mass ratio, where proteins are denatured and coated with a negative charge, allowing them to migrate through a polyacrylamide gel matrix under the influence of an electric field.

**Why the Correct Answer is Right**
Proteins in SDS-PAGE are separated based on their size, with smaller proteins migrating faster towards the anode. This is because the SDS detergent coats the proteins with a uniform negative charge, eliminating any charge effects. The molecules then move through the gel matrix due to their size, with smaller molecules creating less resistance and moving more quickly. The polyacrylamide gel matrix provides a uniform resistance to protein movement, making size the primary factor in separation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** While charge is a factor in some protein separation techniques, it is not the primary factor in SDS-PAGE due to the uniform negative charge imparted by SDS.
* **Option C:** This option is not specifically related to the separation mechanism of SDS-PAGE.
* **Option D:** This option is not correct, as size is the primary factor in SDS-PAGE separation.

**Clinical Pearl / High-Yield Fact**
In SDS-PAGE, the addition of SDS detergent denatures proteins, allowing them to be separated based on size rather than native charge. This technique is widely used in molecular biology and biochemistry for protein analysis.

**Correct Answer:** C. Size