**Core Concept**
Alpha-fetoprotein (AFP) is a major plasma protein produced by the yolk sac and the fetal liver during pregnancy. It is a member of the albuminoid family and plays a crucial role in fetal development.

**Why the Correct Answer is Right**
AFP is synthesized by the fetal yolk sac and liver as a response to the need for protein production in the fetus. The production of AFP is mediated by the gene AFP, which is responsible for coding the protein. The AFP protein is then secreted into the maternal circulation through the placenta, where it can be measured as a marker of fetal health.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because AFP is not synthesized by the placenta itself, but rather by the fetal yolk sac and liver.
**Option B:** This option is incorrect because AFP is not a hormone, but rather a protein produced by the fetal yolk sac and liver.
**Option C:** This option is incorrect because AFP is not synthesized by the mother's liver, but rather by the fetal liver.

**Clinical Pearl / High-Yield Fact**
AFP is an important marker of fetal health, and elevated levels can indicate fetal anomalies or liver disease. A normal AFP level is essential for a healthy pregnancy outcome.

**Correct Answer: D. Fetal yolk sac and liver.**