**Core Concept**
Estrogen production in postmenopausal women is significantly reduced compared to premenopausal women. The primary source of estrogen in postmenopausal women is not the ovaries, which cease to produce estrogen after menopause. Instead, estrogen is produced peripherally through the conversion of androgens.

**Why the Correct Answer is Right**
The correct answer is **A. Peripheral conversion of androgens**. This process involves the enzyme aromatase, which converts androstenedione and testosterone into estrone and estradiol, respectively. These peripheral estrogens are produced in various tissues, including adipose tissue, liver, and bone marrow. This peripheral conversion is the major source of estrogen in postmenopausal women, contributing to the maintenance of estrogen levels in the body.

**Why Each Wrong Option is Incorrect**
* **Option B:** The adrenal glands produce small amounts of estrogen, but they are not the major source of estrogen in postmenopausal women.
* **Option C:** The placenta is a significant source of estrogen during pregnancy, but it is not relevant in postmenopausal women.
* **Option D:** The ovaries are the primary source of estrogen in premenopausal women, but they cease to produce significant amounts of estrogen after menopause.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that peripheral conversion of androgens to estrogens is a crucial mechanism in postmenopausal women, contributing to the development of estrogen-dependent conditions, such as osteoporosis and certain types of breast cancer.

**Correct Answer:** A. Peripheral conversion of androgens.