**Core Concept**
Tumor markers are substances produced by cancer cells or in response to cancer that can be found in higher-than-normal amounts in the blood, urine, or tissues of some patients with certain types of cancer. They are used for early detection, diagnosis, and monitoring of cancer progression.

**Why the Correct Answer is Right**
The correct answer is a protein that is often elevated in the serum of patients with certain types of cancer, particularly hepatocellular carcinoma and germ cell tumors. It is produced by the tumor cells themselves or by the liver in response to tumor products. The protein is a glycoprotein that is secreted into the bloodstream and can be detected using immunological assays. **Alpha-fetoprotein (AFP)** is the primary tumor marker for hepatocellular carcinoma.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because it is not a tumor marker. It is a type of protein that is involved in blood clotting.
**Option B:** This is incorrect because it is not a tumor marker. It is a type of enzyme that is involved in glucose metabolism.
**Option C:** This is incorrect because it is not a tumor marker. It is a type of protein that is involved in immune function.

**Clinical Pearl / High-Yield Fact**
Alpha-fetoprotein (AFP) is a tumor marker that can be used to monitor the response to treatment in patients with hepatocellular carcinoma. A decrease in AFP levels can indicate a good response to therapy, while an increase in AFP levels can indicate tumor progression.

**Correct Answer:** C.