**Core Concept**
Oral contraceptives containing ethinyl oestradiol and a progesterone, also known as combined oral contraceptives (COCs), primarily work by disrupting the normal menstrual cycle and preventing ovulation. This is achieved through a complex interplay of hormonal fluctuations, which ultimately leads to a decrease in the production of gonadotropin-releasing hormone (GnRH) from the hypothalamus.

**Why the Correct Answer is Right**
The mechanism of action of COCs involves the suppression of GnRH secretion, which in turn inhibits the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland. This reduction in LH and FSH levels prevents the maturation of follicles in the ovaries, thereby preventing ovulation. Additionally, the progesterone component of COCs works to further suppress LH secretion, creating a negative feedback loop that maintains anovulation. The combination of ethinyl oestradiol and a progesterone also helps to thicken cervical mucus, making it more difficult for sperm to penetrate the uterus, but this is not the primary mechanism of action.

**Why Each Wrong Option is Incorrect**
**Option B:** While the progesterone component of COCs may have some effect on implantation, this is not the primary mechanism of action. The progesterone component primarily works to maintain the endometrial lining, making it less receptive to implantation, but this is not the main reason for the contraceptive effect.

**Option C:** Thickening of cervical mucus is a secondary effect of COCs, which helps to prevent sperm from reaching the uterus. However, this is not the primary mechanism of action, and the correct answer is the inhibition of ovulation.

**Option D:** This option is incorrect because the correct answer is indeed one of the options above, specifically inhibiting ovulation.

**Clinical Pearl / High-Yield Fact**
It is essential for healthcare providers to remember that the primary mechanism of action of COCs is the inhibition of ovulation, and not the thickening of cervical mucus or the prevention of implantation. This knowledge can help guide patient counseling and ensure that patients understand the correct mechanism of action of their medication.

**✓ Correct Answer: A. Inhibiting ovulation**