**Core Concept**
The differentiation of the genital ridge is a critical process in embryonic development that determines the formation of the gonads, which are essential for sex determination and the development of secondary sex characteristics. This process involves the coordinated action of various genes and hormones, ultimately leading to the formation of either ovaries or testes.

**Why the Correct Answer is Right**
The differentiation of the genital ridge occurs around 6-7 weeks of gestation, when the presence or absence of the SRY gene on the Y chromosome triggers the expression of Sox9 and other genes that promote testis development. In the absence of the SRY gene, the genital ridge differentiates into ovaries under the influence of anti-Müllerian hormone (AMH) and estrogen. The correct answer is the time frame during which this differentiation occurs.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because it does not specify a time frame, and the differentiation of the genital ridge is a specific event that occurs during embryonic development.
**Option B:** This is incorrect because it is not a specific time frame, and the differentiation of the genital ridge occurs before the formation of the external genitalia.
**Option C:** This is incorrect because it is not a specific time frame, and the differentiation of the genital ridge occurs before the formation of the external genitalia.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the presence or absence of the SRY gene on the Y chromosome determines the sex of the embryo, and this is the critical event that triggers the differentiation of the genital ridge into either ovaries or testes.

**Correct Answer:** C. 6-7 weeks of gestation.