**Core Concept**
Natural estrogens are biologically active compounds produced by the human body, primarily by the ovaries, placenta, and adipose tissue. They play crucial roles in reproductive and sexual health, including regulation of the menstrual cycle, fertility, and development of female secondary sexual characteristics.

**Why the Correct Answer is Right**
Estrogens are a group of steroid hormones that exert their effects through binding to estrogen receptors. The main natural estrogens produced by the human body are estrone (E1), estradiol (E2), and estriol (E3). Estradiol is the most potent and prevalent natural estrogen, playing a critical role in reproductive and sexual health. The other options listed (E1 and E3) are also natural estrogens, but their levels and activities are generally lower than those of estradiol.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because it is not a natural estrogen. While it is related to estrogen, it is not a naturally occurring compound in the human body.
**Option C:** This option is incorrect because it is a synthetic estrogen, not a natural one. It is a man-made compound used in various medical and cosmetic applications.

**Clinical Pearl / High-Yield Fact**
It's essential to remember the main natural estrogens produced by the human body: estrone (E1), estradiol (E2), and estriol (E3). These compounds play critical roles in reproductive and sexual health, and their dysregulation can lead to various disorders, including infertility, menstrual irregularities, and breast cancer.

**Correct Answer: D.**