**Core Concept**
The ovarian reserve test assesses the quantity and quality of a woman's remaining oocytes (eggs) in the ovaries, which is crucial for evaluating fertility potential, especially in women with a history of infertility or those undergoing fertility preservation.

**Why the Correct Answer is Right**
The most commonly used test for ovarian reserve is the Anti-Müllerian Hormone (AMH) assay. AMH is a protein produced by the granulosa cells in small antral follicles in the ovaries. Its levels reflect the number of antral follicles present in the ovaries and, by extension, the ovarian reserve. Elevated AMH levels indicate a higher number of antral follicles, suggesting a good ovarian reserve, whereas low AMH levels suggest a poor ovarian reserve.

**Why Each Wrong Option is Incorrect**
**Option A:** While the Follicle-Stimulating Hormone (FSH) level can provide some information about ovarian reserve, it is not the most reliable or commonly used test. Elevated FSH levels can indicate a poor ovarian reserve, but this test is less sensitive and more variable than the AMH assay.

**Option B:** The basal antral follicle count (BAFC) is a more invasive and subjective method of assessing ovarian reserve, which involves counting the number of antral follicles in the ovaries using ultrasound. While it can provide useful information, it is not as widely available or as easy to perform as the AMH assay.

**Option C:** The clomiphene citrate challenge test (CCCT) involves measuring FSH levels before and after administration of clomiphene citrate. While it can provide some information about ovarian reserve, it is less commonly used and less reliable than the AMH assay.

**Clinical Pearl / High-Yield Fact**
A low AMH level (<1.1 ng/mL) is often used as a cutoff to indicate a poor ovarian reserve, but it is essential to consider other factors, such as age and menstrual cycle irregularities, when interpreting AMH results. **Correct Answer:** B. Basal antral follicle count (BAFC)