**Core Concept**
Krukenberg tumors are a type of metastatic tumor that involves the ovaries, typically originating from a primary site in the gastrointestinal tract. The ovaries are seeded with tumor cells, which then undergo a process called "metastatic dedifferentiation," resulting in a distinctive mucin-producing, signet-ring cell morphology.

**Why the Correct Answer is Right**
The primary site of Krukenberg tumors is most commonly the stomach, particularly the gastric mucosa. The tumor cells from the primary site migrate to the ovaries through the bloodstream or lymphatic system, where they implant and grow. The signet-ring cells in Krukenberg tumors are characterized by their abundant mucin production and peripheral nuclei, which are a result of the metastatic dedifferentiation process.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because while other gastrointestinal sites such as the colon and small intestine can be primary sites for Krukenberg tumors, the stomach is the most common and well-documented primary site.
**Option B:** This is incorrect because Krukenberg tumors are not typically associated with the breast as a primary site.
**Option C:** This is incorrect because while the pancreas can be a primary site for metastatic tumors, it is not a common primary site for Krukenberg tumors.

**Clinical Pearl / High-Yield Fact**
Krukenberg tumors are often bilateral and can be difficult to distinguish from primary ovarian carcinomas, highlighting the importance of thorough clinical and pathological evaluation, including examination of the primary site and review of the patient's medical history.

**Correct Answer: D. Stomach.**