**Core Concept**
Protein purification and separation are crucial steps in the production of therapeutic proteins, vaccines, and other biomedical products. Various techniques are employed to isolate and characterize proteins from complex biological mixtures, including cell lysates, tissues, and bodily fluids.

**Why the Correct Answer is Right**
Protein purification techniques often involve a combination of physical and chemical methods to separate proteins based on their physical and chemical properties. Techniques such as Chromatography (e.g., ion exchange, size exclusion, and affinity chromatography), Electrophoresis (e.g., SDS-PAGE and Western blotting), and Ultrafiltration are commonly used for protein purification.

**Why Each Wrong Option is Incorrect**
**Option A:** Chromatography is a widely used technique in protein purification, so it is not the correct answer.
**Option B:** Electrophoresis is another essential method for separating proteins based on their charge, size, and other properties, making it incorrect as well.
**Option C:** Ultrafiltration is a technique used to separate proteins based on their molecular weight, which is a common method in protein purification.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that protein purification techniques often involve a combination of multiple methods to achieve high purity and yield. The choice of technique depends on the specific protein properties, the scale of production, and the desired level of purity.

**Correct Answer:** D.