**Core Concept**
Primordial follicles are the earliest stage of follicular development in the ovaries, consisting of an oocyte surrounded by a single layer of flat granulosa cells. The development of primordial follicles is a critical process in female reproductive biology, and understanding their formation and maturation is essential for grasping ovarian function.

**Why the Correct Answer is Right**
Primordial follicles are formed during fetal development, specifically between 12 to 20 weeks of gestation. The process of follicular development is tightly regulated by a complex interplay of genetic and hormonal factors. The oocyte within the primordial follicle undergoes rapid growth and maturation, eventually giving rise to a pre-antral follicle. This process is crucial for the production of a pool of follicles that will be available for ovulation and fertilization during reproductive life.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as the development of primordial follicles is not complete by the end of fetal development.
**Option B:** Incorrect, as the exact timing of primordial follicle development is not accurately described.
**Option C:** Incorrect, as the development of primordial follicles is a complex process that occurs over a longer period of time.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that primordial follicle development is a critical window for ovarian reserve establishment, and disruptions during this period can lead to long-term consequences for reproductive health.

**Correct Answer: C. 20 weeks of gestation.**