**Core Concept**
The number of primordial follicles in the ovary at birth is a critical aspect of ovarian reserve and reproductive physiology. The primordial follicles are the resting stage of follicular development, containing an oocyte surrounded by a layer of flat granulosa cells. The initial number of primordial follicles sets the stage for ovarian function throughout life.

**Why the Correct Answer is Right**
The correct number of primordial follicles at birth is approximately 1-2 million. This large pool of follicles is gradually depleted over a woman's lifetime due to follicular recruitment, growth, and atresia. The rate of follicular depletion is influenced by various factors, including genetics, environmental factors, and hormonal influences. The primordial follicles are present in the ovarian cortex, and their number is determined by the balance between follicular recruitment and atresia.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect because the actual number of primordial follicles at birth is higher than 0.5 million. A number this low would imply a significant reduction in ovarian reserve from the time of birth.
* **Option B:** This option may be tempting due to its proximity to the correct answer, but it is still too low. The correct number is actually higher than 1 million.
* **Option D:** This option is too high and may be an overestimation of the initial ovarian reserve. While the number of primordial follicles does decrease over time, the initial pool is substantial but not as large as this option suggests.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the number of primordial follicles at birth is a critical determinant of ovarian reserve and reproductive potential. Factors that deplete the primordial follicle pool, such as chemotherapy or premature ovarian failure, can significantly impact a woman's ability to conceive.

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