**Core Concept**
Krukenberg tumor refers to a type of metastatic tumor in the ovary, characterized by mucin-producing signet-ring cells. This phenomenon is a result of the spread of malignant cells from a primary site, typically a gastrointestinal cancer, to the ovaries.

**Why the Correct Answer is Right**
The most common primary site for Krukenberg tumors is gastric adenocarcinoma. The gastric cancer cells metastasize to the ovaries through hematogenous or lymphatic spread, leading to the formation of Krukenberg tumors. The signet-ring cells in the ovary are a result of the compression of the cell contents against the cell membrane due to the accumulation of mucin.

**Why Each Wrong Option is Incorrect**
**Option A:** While breast cancer can metastasize to the ovaries, it is not the most common primary site for Krukenberg tumors.
**Option B:** Colorectal cancer is a possible primary site for Krukenberg tumors, but it is less common than gastric adenocarcinoma.
**Option C:** Other gastrointestinal cancers, such as pancreatic cancer, can also metastasize to the ovaries, but they are not the most common primary site for Krukenberg tumors.

**Clinical Pearl / High-Yield Fact**
Krukenberg tumors can mimic primary ovarian cancers, making it essential to perform a thorough metastatic workup, including imaging studies and biopsies, to determine the primary site of the tumor.

**Correct Answer: D. Gastric adenocarcinoma.**