**Core Concept**
CA-125 is a tumor marker that is associated with various conditions, primarily used in the monitoring and diagnosis of ovarian cancer. It is a mucin-type glycoprotein that is overexpressed in the serum of patients with epithelial ovarian cancer.

**Why the Correct Answer is Right**
CA-125 is a glycoprotein that is produced by the epithelial cells of the ovary. In ovarian cancer, especially the serous type, CA-125 is overexpressed and released into the bloodstream, leading to elevated serum levels. This makes CA-125 a useful marker for monitoring disease progression and response to treatment in patients with ovarian cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** CA-125 is not specific for breast cancer, although it can be elevated in some cases of breast cancer. However, its sensitivity and specificity are not sufficient to make it a reliable marker for breast cancer.
**Option B:** CA-125 is not specific for colon cancer, although it can be elevated in some cases of colon cancer. Other markers, such as CEA, are more specific for colon cancer.
**Option C:** CA-125 is not specific for lung cancer, although it can be elevated in some cases of lung cancer. Other markers, such as CYFRA 21-1, are more specific for lung cancer.

**Clinical Pearl / High-Yield Fact**
CA-125 levels can be elevated in benign conditions such as endometriosis, pelvic inflammatory disease, and menstruation, which can lead to false-positive results.

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