**Core Concept**
Alpha-fetoprotein (AFP) is a major plasma protein produced by the yolk sac and the fetal liver during fetal development. It serves as a tumor marker for certain types of cancer, particularly those of hepatocellular and germ cell origins.

**Why the Correct Answer is Right**
AFP is used as a tumor marker because its levels are elevated in the serum of individuals with hepatocellular carcinoma, non-seminomatous germ cell tumors, and certain other cancers. The mechanism behind this elevation is not fully understood, but it is thought to be related to the abnormal expression of genes regulating AFP production in tumor cells. Elevated AFP levels can aid in the diagnosis and monitoring of these cancers, particularly in cases where imaging studies are inconclusive.

**Why Each Wrong Option is Incorrect**
**Option A:** AFP is not a marker for breast cancer. While some breast cancers may have elevated AFP levels, it is not a specific or sensitive marker for this disease.
**Option B:** AFP is not a marker for lung cancer. Although some lung cancers may have elevated AFP levels, it is not a common or reliable marker for this disease.
**Option C:** AFP is not a marker for colon cancer. While some colon cancers may have elevated AFP levels, it is not a specific or sensitive marker for this disease.

**Clinical Pearl / High-Yield Fact**
It is essential to note that elevated AFP levels can also be seen in normal pregnancy, liver disease, and other non-cancerous conditions. Therefore, a diagnosis of cancer based solely on elevated AFP levels requires further evaluation and confirmation.

**Correct Answer: C. Non-seminomatous germ cell tumors and hepatocellular carcinoma.**