**Core Concept**
Alpha-fetoprotein (AFP) is a major plasma protein produced by the yolk sac and the fetal liver during fetal development. Elevated levels of AFP can be associated with various conditions, including certain types of cancer. In the context of tumors, AFP is a tumor marker that can be used to monitor disease progression or recurrence.

**Why the Correct Answer is Right**
The correct answer is associated with hepatocellular carcinoma (HCC), a type of liver cancer. In HCC, AFP is produced by the tumor cells and released into the bloodstream, leading to elevated levels. This is because AFP is a protein that is normally produced by the fetal liver, and in some cases, cancer cells can re-express this protein. The production of AFP is often correlated with the aggressiveness of the tumor and can be used as a prognostic indicator.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because AFP is not typically elevated in thyroid cancer. While AFP can be elevated in certain types of germ cell tumors, which can occur in the testes or ovaries, thyroid cancer is not associated with elevated AFP levels.
**Option B:** This option is incorrect because AFP is not typically elevated in breast cancer. While there are various tumor markers associated with breast cancer, AFP is not one of them.
**Option C:** This option is incorrect because AFP is not typically elevated in lung cancer. While there are various tumor markers associated with lung cancer, AFP is not one of them.

**Clinical Pearl / High-Yield Fact**
AFP can be used as a tumor marker to monitor disease progression or recurrence in patients with HCC. However, it's essential to note that AFP levels can also be elevated in other conditions, such as liver regeneration, pregnancy, and certain types of infections.

**Correct Answer: D. Hepatocellular carcinoma.**