**Core Concept**
The syncytiotrophoblast is a critical layer of cells that forms the maternal-fetal interface in the placenta, facilitating the exchange of nutrients, gases, and waste products between the mother and the fetus. To maintain this delicate balance, the syncytiotrophoblast is separated from the fetal blood by a layer of cells, which helps to regulate the passage of substances.

**Why the Correct Answer is Right**
The correct layer separating the fetal blood from the syncytiotrophoblast is the cytotrophoblast. The cytotrophoblast is a layer of undifferentiated trophoblast cells that lies beneath the syncytiotrophoblast, forming a barrier between the fetal blood and the maternal-fetal interface. This layer helps to regulate the exchange of substances and maintains the integrity of the placental barrier.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the cytotrophoblast is the correct layer separating the fetal blood from the syncytiotrophoblast, not the trophoblastic basement membrane.
* **Option B:** This option is incorrect because the chorionic villi are finger-like projections that contain blood vessels and provide a large surface area for gas exchange, but they are not directly involved in separating the fetal blood from the syncytiotrophoblast.
* **Option D:** This option is incorrect because the maternal blood is separated from the syncytiotrophoblast by the trophoblastic basement membrane, not the fetal blood.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the cytotrophoblast plays a crucial role in maintaining the integrity of the placental barrier, and any disruption to this layer can lead to complications during pregnancy.

**Correct Answer:** C. Chorionic villi.