**Core Concept**
Epithelial ovarian cancer is a type of cancer originating from the epithelial cells covering the ovary. Tumor markers are substances produced by cancer cells or in response to cancer that can be measured in the blood. The most helpful tumor marker in the follow-up of epithelial ovarian cancer is one that is specifically elevated in this type of cancer and can indicate recurrence or progression.

**Why the Correct Answer is Right**
CA-125 (cancer antigen 125) is a glycoprotein produced by the epithelial cells of the ovary and is commonly elevated in epithelial ovarian cancer. It is used as a tumor marker to monitor the response to treatment and detect recurrence. Elevated CA-125 levels can indicate disease progression or spread, while decreasing levels can suggest a positive response to treatment. CA-125 is not specific to ovarian cancer and can be elevated in other conditions, but it is the most commonly used tumor marker in the follow-up of epithelial ovarian cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it is not a commonly used tumor marker for epithelial ovarian cancer. While it may be elevated in some cases, it is not as specific or sensitive as CA-125.
**Option B:** This option is incorrect as it is a tumor marker for testicular cancer, not ovarian cancer. It is produced by the germ cells of the testis and is used to monitor the response to treatment and detect recurrence in testicular cancer.
**Option B:** This option is incorrect as it is a tumor marker for colorectal cancer, not ovarian cancer. It is elevated in the stool of patients with colorectal cancer and can be used to screen for the disease.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while CA-125 is the most helpful tumor marker in the follow-up of epithelial ovarian cancer, it is not specific to this disease and can be elevated in other conditions. A rising CA-125 level in a patient with a history of ovarian cancer can indicate recurrence or progression, but a normal CA-125 level does not rule out recurrence.

**Correct Answer:** C. CA-125.