**Core Concept**
CA-125 is a tumor marker primarily used in the monitoring and follow-up of ovarian cancer. It is a mucin-type glycoprotein that is often elevated in the serum of patients with epithelial ovarian cancer, particularly in the advanced stages.

**Why the Correct Answer is Right**
CA-125 is not specific to ovarian cancer and can be elevated in various other conditions, including endometriosis, pelvic inflammatory disease, and benign ovarian tumors. However, its elevation is a common finding in epithelial ovarian cancer, especially in the postmenopausal population. The CA-125 antigen is expressed on the surface of ovarian cancer cells, and its levels can be used as a prognostic indicator and to monitor the response to treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** Endometriosis can cause elevated CA-125 levels, but it is not the primary condition associated with this tumor marker.
**Option B:** Pelvic inflammatory disease can lead to inflammation and elevated CA-125 levels, but it is not a primary cause of elevated CA-125 in the context of ovarian cancer.
**Option C:** Benign ovarian tumors can cause elevated CA-125 levels, but they are not as strongly associated with CA-125 elevation as epithelial ovarian cancer.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that CA-125 is not a screening test for ovarian cancer, as many false positives can occur due to other benign conditions. It's primarily used in the monitoring and follow-up of patients with known ovarian cancer.

**Correct Answer:** C. Benign ovarian tumors.