**Core Concept**
CNS development begins with the formation of the neural plate, which eventually undergoes neurogenesis, the process of neural cell proliferation and differentiation. This complex process involves the coordinated action of various signaling pathways and transcription factors.

**Why the Correct Answer is Right**
The correct answer is week 3 of gestation, when the neural plate forms as an outgrowth of the ectoderm. This is followed by the process of neurulation, where the neural plate invaginates to form the neural tube, which will eventually give rise to the brain and spinal cord. The neural tube closes around the 26th day of gestation, marking the beginning of CNS development.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because CNS development begins before the embryonic period, specifically during the third week of gestation.
**Option B:** This is incorrect because while the neural plate forms in week 3, the closure of the neural tube marks the beginning of CNS development, not the completion of it.
**Option C:** This is incorrect because while the neural tube closes in week 3, the process of CNS development is far from complete at this stage, with significant growth and differentiation occurring in the subsequent weeks.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the closure of the neural tube is a critical period in CNS development, as defects in this process can lead to neural tube defects, such as spina bifida or anencephaly.

**Correct Answer: C. 3 weeks.**