**Core Concept**
The establishment of placental circulation is a critical event in embryonic development, marking the transition from a hemochorial to a hemotrophic mode of nutrition. This process involves the invasion of trophoblast cells into the uterine lining and the formation of a network of blood vessels that facilitate exchange between the mother and the fetus.

**Why the Correct Answer is Right**
Placental circulation is established approximately 10-12 days after ovulation, when the trophoblast cells have invaded the uterine lining and formed a network of blood vessels. This process is mediated by the action of various enzymes and growth factors, including human chorionic gonadotropin (hCG) and vascular endothelial growth factor (VEGF). The establishment of placental circulation marks the beginning of a hemotrophic mode of nutrition, where the fetus relies on the mother's blood supply for oxygen and nutrients.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because placental circulation is established before implantation of the blastocyst, which occurs approximately 6-8 days after fertilization.
* **Option B:** This option is incorrect because while the development of the placenta begins around 10-12 days after ovulation, it is not fully established until later in pregnancy.
* **Option C:** This option is incorrect because the implantation of the blastocyst, not the establishment of placental circulation, is the critical event that occurs around 6-8 days after fertilization.

**Clinical Pearl / High-Yield Fact**
Remember that the establishment of placental circulation is a critical event in embryonic development, and any disruption to this process can have significant consequences for fetal development and pregnancy outcome.

**Correct Answer:** C. 10-12 days