**Core Concept**
Estrogen production in the menstrual cycle is primarily regulated by the **hypothalamic-pituitary-ovarian axis**. The **follicle-stimulating hormone (FSH)** from the pituitary gland stimulates the growth of follicles in the ovary, which then produce **estrogen**.

**Why the Correct Answer is Right**
The correct answer choice is related to the fact that estrogen levels surge during the **follicular phase** due to the growth of follicles under the influence of FSH. This increase in estrogen levels helps in the thickening of the **endometrium** and prepares it for a potential pregnancy. The **estrogen** produced by the follicles also plays a crucial role in the negative and positive feedback mechanisms regulating the menstrual cycle.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not accurately describe the primary source or regulation of estrogen production in the menstrual cycle.
**Option B:** Incorrect as it might refer to a different aspect of the menstrual cycle or hormone regulation.
**Option C:** Incorrect because, although related to the menstrual cycle, it does not specifically address estrogen production.
**Option D:** Incorrect as it might pertain to a different hormone or phase of the menstrual cycle.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **estrogen** levels are highest right before **ovulation**, which typically occurs around the midpoint of a 28-day menstrual cycle. Understanding the fluctuations in estrogen levels is crucial for managing conditions like **infertility** and **hormone-sensitive cancers**.

**Correct Answer:** D. Estrogen is produced by the follicle.