**Core Concept**
The question requires knowledge of fetal hematopoiesis, specifically the initial site of red blood cell (RBC) production in the fetus. Hematopoiesis is the process of blood cell formation, and it occurs in different sites in the fetus at various stages of development.

**Why the Correct Answer is Right**
The yolk sac is the primary site of RBC production in the early fetus, typically from around 12-16 days post-fertilization until approximately 8 weeks of gestation. This is due to the presence of hematopoietic cells, including erythroblast islands, which are clusters of erythroid precursor cells that eventually give rise to mature RBCs. The yolk sac provides a nutrient-rich environment that supports the growth and differentiation of these cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** The gestational sac is the earliest structure visible on ultrasound, but it is not a site of RBC production. It is a fluid-filled sac that surrounds the embryo and provides protection during early development.
* **Option C:** The placenta is a vital organ that provides oxygen and nutrients to the fetus and removes waste products. While it does contain blood vessels and cells, it is not the primary site of RBC production in the fetus.
* **Option D:** Fetal bones are involved in hematopoiesis later in development, typically from around 24 weeks of gestation onwards. However, they are not the initial site of RBC production.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the yolk sac is a transient structure that is eventually absorbed into the body of the fetus. Understanding the sequence of hematopoietic sites in the fetus is crucial for diagnosing and managing conditions related to abnormal blood cell production.

✓ Correct Answer: B. Yolk sac