**Core Concept**
Papillary serous carcinoma of the ovary is a type of epithelial ovarian cancer, accounting for approximately 70% of ovarian cancer cases. This malignancy is characterized by the production of serous fluid, which is often elevated in tumor markers.

**Why the Correct Answer is Right**
Cancer antigen 125 (CA-125) is a mucin-like glycoprotein that is overexpressed on the surface of ovarian cancer cells, particularly in serous carcinomas. Elevated levels of CA-125 are associated with tumor recurrence and progression. In the context of ovarian cancer, CA-125 is used as a tumor marker for monitoring disease recurrence, response to treatment, and disease progression. The serum level of CA-125 can be elevated in various conditions, but in the context of ovarian cancer, a persistently elevated level is indicative of recurrence.

**Why Each Wrong Option is Incorrect**
**Option A:** Human chorionic gonadotropin (hCG) is a hormone produced during pregnancy, and elevated levels are often seen in choriocarcinoma, a type of gestational trophoblastic disease. It is not typically used as a tumor marker for ovarian cancer.

**Option B:** Alpha-fetoprotein (AFP) is a protein produced by the yolk sac and liver during fetal development. Elevated levels of AFP are often seen in hepatocellular carcinoma and certain germ cell tumors, such as yolk sac tumors. It is not typically used as a tumor marker for ovarian cancer.

**Option C:** Human epidermal growth factor receptor 2 (HER2) is a receptor protein that is often overexpressed in certain types of breast cancer and gastric cancer. While HER2 is a relevant biomarker in these cancers, it is not typically used as a tumor marker for ovarian cancer.

**Option D:** Prostate-specific antigen (PSA) is a protein produced by the prostate gland, and elevated levels are often seen in prostate cancer. PSA is not typically used as a tumor marker for ovarian cancer.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while CA-125 is a useful tumor marker for ovarian cancer, it is not specific to ovarian cancer and can be elevated in various benign conditions, such as pelvic inflammatory disease, endometriosis, and pregnancy. Therefore, a persistently elevated level of CA-125 in a patient with a history of ovarian cancer should prompt further investigation for recurrence.

**Correct Answer:** C. Cancer antigen 125 (CA-125).