**Core Concept**
The question is testing the understanding of embryonic development, specifically the formation of the blood islands in the yolk sac during the third week of gestation. Blood islands are crucial for the development of the hematopoietic system, and the cells involved in their formation are key to understanding the origin of blood cells.

**Why the Correct Answer is Right**
The correct answer is related to the hematopoietic process, where mesenchymal cells aggregate to form blood islands. These blood islands are the site of primitive blood vessel formation and hematopoiesis. The mesenchymal cells involved in this process differentiate into angioblasts, which eventually give rise to endothelial cells lining the blood vessels. **Option C** is correct because it refers to the group of mesenchymal cells that aggregate to form blood islands.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specifically refer to the group of mesenchymal cells involved in blood island formation.
* **Option B:** This option is incorrect because it refers to a different process, where hematopoietic cells migrate to the fetal liver and spleen for further development.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the development of the hematopoietic system is a complex process, and understanding the origin of blood cells is crucial for diagnosing and treating hematological disorders. The formation of blood islands in the yolk sac is a critical step in this process.

**Correct Answer: C. Mesoblasts**