**Core Concept**
Progesterone-only pills (POPs) are a type of hormonal contraceptive that primarily work by inhibiting ovulation, thickening cervical mucus to prevent sperm penetration, and altering the endometrium to prevent implantation of a fertilized egg. They are also known as mini-pills or progestin-only pills.

**Why the Correct Answer is Right**
Progesterone-only pills work by inhibiting the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which in turn reduces the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland. This reduction in LH and FSH leads to a decrease in estrogen production by the ovaries, ultimately resulting in anovulation. Additionally, POPs increase the thickness of cervical mucus, making it more difficult for sperm to penetrate, and alter the endometrium, making it less receptive to implantation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because POPs do not primarily work by increasing the risk of thromboembolic events, although they may have a slightly higher risk compared to combined oral contraceptives.
* **Option B:** This option is incorrect because POPs do not primarily work by reducing the risk of endometrial cancer, although they may have a protective effect against this condition.
* **Option C:** This option is incorrect because POPs do not primarily work by increasing the risk of breast cancer, although the relationship between POPs and breast cancer risk is still being studied.

**Clinical Pearl / High-Yield Fact**
It's essential to note that POPs can be used in women who are breastfeeding, as they do not affect milk production, making them a good option for lactating women who want to use hormonal contraception.

**Correct Answer: A. Inhibit ovulation.**