**Core Concept**
Erythropoietin (EPO) is a hormone that regulates red blood cell production. In the fetus, the production of EPO is crucial for erythropoiesis, the process of red blood cell development. The major site of EPO production during fetal development is a key concept in understanding the regulation of red blood cell production in the fetus.

**Why the Correct Answer is Right**
During fetal development, the major site of EPO production is the **yolk sac**. The yolk sac is a temporary organ that provides nutrients to the embryo before the placenta develops. The yolk sac produces EPO, which stimulates the production of red blood cells in the fetal liver and spleen. This process is essential for the fetus to adapt to the changing oxygen demands during development.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although the kidneys are a major site of EPO production in adults, they are not the primary site during fetal development.
* **Option B:** The liver is involved in erythropoiesis during fetal development, but it is not the major site of EPO production.
* **Option D:** The placenta is involved in the transfer of oxygen and nutrients from the mother to the fetus but is not the major site of EPO production.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the yolk sac is a temporary organ that plays a critical role in fetal development, including the production of EPO. This knowledge is crucial for understanding the regulation of red blood cell production in the fetus.

**Correct Answer:** C.