**Core Concept**
The number of primordial follicles present in the human ovary at birth is a critical aspect of ovarian reserve. These follicles are the primary source of oocytes throughout a woman's reproductive life. The initial count of primordial follicles determines the potential number of eggs a woman can produce.

**Why the Correct Answer is Right**
At birth, the human ovary contains approximately 1-2 million primordial follicles. These follicles are formed during fetal development and are present in the ovary from birth. The primordial follicles are composed of an oocyte surrounded by a layer of flat granulosa cells. The oocytes remain in a dormant state until puberty, when follicular growth and ovulation begin. The number of primordial follicles at birth is a reflection of the ovarian reserve, which declines with age due to follicular depletion.

**Why Each Wrong Option is Incorrect**
* **Option A:** There is no scientific evidence to support the claim that the number of primordial follicles at birth is 5 million. This option may be based on outdated information or incorrect assumptions.
* **Option B:** The number of primordial follicles at birth is not 1 million, but rather a range of 1-2 million. This option is an underestimation of the actual number.
* **Option D:** There is no credible source that suggests the number of primordial follicles at birth is 0.5 million. This option is an oversimplification and does not accurately reflect the current understanding of ovarian development.

**Clinical Pearl / High-Yield Fact**
The number of primordial follicles at birth is a critical determinant of ovarian reserve, which declines with age due to follicular depletion. This knowledge is essential for understanding the reproductive physiology and clinical implications of ovarian aging.

**Correct Answer: C. 1-2 million.**