**Core Concept**
During the menstrual cycle, a series of hormonal changes regulate ovulation and prepare the uterus for potential pregnancy. One of these hormonal changes involves an increase in body temperature, which is influenced by the levels of estrogen and progesterone.

**Why the Correct Answer is Right**
Progesterone is known to cause an increase in body temperature due to its effect on the hypothalamus. This is mediated through the action of progesterone on the hypothalamic thermoregulatory centers, leading to an increase in the set point for body temperature. As a result, the basal body temperature rises, typically by 0.4-1.0°C, during the luteal phase of the menstrual cycle. This increase in body temperature is a classic sign of ovulation and is often used as a method of natural family planning.

**Why Each Wrong Option is Incorrect**
**Option A:** Estrogen levels actually decrease during ovulation, and while estrogen does have some effect on body temperature, it is not the primary hormone responsible for the increase.

**Option C:** LH (luteinizing hormone) is involved in ovulation, but it does not directly cause an increase in body temperature. Instead, LH triggers the release of progesterone from the corpus luteum, which then causes the increase in body temperature.

**Option D:** FSH (follicle-stimulating hormone) is also involved in ovulation, but it does not directly influence body temperature.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the increase in basal body temperature is a subtle change, and a rise of 0.4-1.0°C can be difficult to detect. Therefore, it's crucial to use a basal body thermometer to accurately track changes in body temperature during the menstrual cycle.

**✓ Correct Answer: B. Progesterone. Progesterone is the hormone responsible for the increase in body temperature during ovulation.**