**Core Concept**
Oral contraceptives are a type of hormonal medication used to prevent pregnancy. They primarily act by regulating ovulation, thickening cervical mucus, and thinning the uterine lining, thereby preventing fertilization and implantation.

**Why the Correct Answer is Right**
Oral contraceptives contain a combination of estrogen and progesterone, which work synergistically to inhibit the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. This leads to a decrease in the production of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland, resulting in the suppression of ovulation. Additionally, the progestin component of oral contraceptives causes changes in the cervical mucus, making it more difficult for sperm to penetrate the uterus, and also thins the endometrial lining, making it unfavorable for implantation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because oral contraceptives do not primarily act by inhibiting the action of oxytocin, which is involved in milk letdown and uterine contractions during childbirth.
* **Option B:** This option is incorrect because oral contraceptives do not primarily act by stimulating the release of GnRH, which would actually increase LH and FSH production and lead to ovulation.
* **Option C:** This option is incorrect because oral contraceptives do not primarily act by blocking the action of prostaglandins, which are involved in various bodily functions including inflammation and smooth muscle contraction.

**Clinical Pearl / High-Yield Fact**
It's essential to note that oral contraceptives can have varying effects on different women, and some may experience side effects such as nausea, breast tenderness, or mood changes. Additionally, oral contraceptives can interact with other medications, including anticonvulsants, antibiotics, and blood thinners.

**Correct Answer:** D