**Core Concept**
CA-125 is a mucin-type glycoprotein that serves as a tumor marker, primarily associated with epithelial ovarian cancer. It is produced by the epithelial cells of the ovary, fallopian tube, and peritoneum. Elevated levels of CA-125 are often used as a diagnostic and monitoring tool for ovarian cancer, although it lacks specificity.

**Why the Correct Answer is Right**
CA-125 is specifically associated with epithelial ovarian cancer due to its expression by the epithelial cells of the ovary. Elevated levels of CA-125 are often seen in ovarian cancer, particularly in advanced stages. This marker is used in conjunction with imaging studies and other diagnostic tools for the detection and monitoring of ovarian cancer. The CA-125 antigen is a large, heavily glycosylated mucin protein that is shed into the peritoneal fluid and bloodstream, making it detectable through serum assays.

**Why Each Wrong Option is Incorrect**
* **Option A:** CA-125 is not a specific marker for breast cancer. Although some breast cancers may express CA-125, it is not a reliable marker for this condition.
* **Option B:** CA-125 is not a specific marker for colon cancer. While some colon cancers may express CA-125, it is not a specific or sensitive marker for this condition.
* **Option D:** CA-125 is not a specific marker for lung cancer. Although some lung cancers may express CA-125, it is not a reliable marker for this condition.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that CA-125 lacks specificity and can be elevated in various benign conditions, such as menstruation, pelvic inflammatory disease, and abdominal surgery. Therefore, a diagnosis of ovarian cancer should not be based solely on elevated CA-125 levels.

**Correct Answer: C. Ovarian cancer.**