**Core Concept**
The molecular weight of a protein can be determined by various methods, which are essential in understanding the structure and function of proteins. These methods involve different techniques that exploit the physical and chemical properties of proteins.

**Why the Correct Answer is Right**
Sedimentation equilibrium ultracentrifugation (Option C) is a technique used to determine the molecular weight of proteins. In this method, a protein solution is centrifuged at high speed, and the concentration of the protein is measured at different radii from the axis of rotation. By analyzing the concentration profile, the molecular weight of the protein can be calculated using the Svedberg equation, which relates the sedimentation coefficient to the molecular weight. This technique is particularly useful for large proteins and complexes.

**Why Each Wrong Option is Incorrect**
* **Option A:** Isoelectric focusing is a technique used to separate proteins based on their isoelectric point, which is the pH at which the protein has no net charge. While it is useful for identifying and separating proteins, it is not used to determine molecular weight.
* **Option B:** Size exclusion chromatography is a technique used to separate proteins based on their size. While it can provide information on the size of proteins, it is not a direct method for determining molecular weight.

**Clinical Pearl / High-Yield Fact**
Remember that the molecular weight of a protein can also be determined by mass spectrometry, which involves ionizing the protein and measuring its mass-to-charge ratio. This technique is particularly useful for small proteins and peptides.

**Correct Answer: C. Sedimentation equilibrium ultracentrifugation**