**Core Concept**
Ovulation is a complex process involving the release of an ovum from the ovary, typically occurring once a month in females of reproductive age. A reliable test of ovulation is crucial for diagnosing and managing fertility issues.

**Why the Correct Answer is Right**
Basal Body Temperature (BBT) is a simple and widely used test to assess ovulation. BBT is measured daily by recording the body temperature at rest, usually first thing in the morning. A slight increase in BBT is observed after ovulation due to the increased production of progesterone by the corpus luteum. This increase in progesterone causes a rise in basal body temperature, which can be used as an indicator of ovulation.

**Why Each Wrong Option is Incorrect**
**Option A:** Mid-cycle luteinizing hormone surge is an indicator of ovulation but is a more complex and less reliable method compared to BBT.
**Option B:** Ultrasound can detect ovulation but is more invasive and expensive compared to BBT.
**Option C:** FSH (Follicle-Stimulating Hormone) levels can indicate ovulation but are not a direct measure of ovulation and can be influenced by various factors.

**Clinical Pearl / High-Yield Fact**
A 0.5-1°C increase in BBT is a reliable indicator of ovulation. This increase typically occurs 2-3 days after ovulation and can be used to predict the fertile window.

**Correct Answer:** C. Ultrasound can detect ovulation but is more invasive and expensive compared to BBT.