**Core Concept**
The number of oocytes present at birth in female gonads is a critical aspect of reproductive biology, influencing female fertility and menstrual cycles. The process of oogenesis and follicular development is essential for understanding this concept.

**Why the Correct Answer is Right**
At birth, the ovaries of a female fetus contain a finite number of oocytes, which are the female germ cells responsible for producing eggs. These oocytes are arrested in prophase I of meiosis and remain in this state until puberty. The number of oocytes present at birth is approximately 1-2 million, but this number declines significantly due to apoptosis and atresia, leaving only about 300,000 to 500,000 oocytes by the time of menarche.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the number of oocytes present at birth is not in the range of 10,000 to 20,000.
* **Option B:** This option is incorrect because the number of oocytes present at birth is not in the range of 5,000 to 10,000.
* **Option C:** This option is incorrect because the number of oocytes present at birth is not in the range of 1 million to 2 million.

**Clinical Pearl / High-Yield Fact**
The decline in the number of oocytes due to apoptosis and atresia is a critical factor in the regulation of female fertility, and understanding this process is essential for the diagnosis and management of reproductive disorders.

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