**Core Concept:** Oral contraceptive pills (OCPs) are a type of hormonal contraceptive that contain estrogen and progesterone hormones, which inhibit ovulation and thicken cervical mucus to prevent sperm from reaching the uterus. Continuous use of hormonal OCPs for 21 days is required to achieve these effects.

**Why the Correct Answer is Right:** To prevent ovulation, OCPs must be taken continuously for a certain period of time. Hormonal OCPs contain estrogen and progesterone hormones that mimic the body's natural hormonal cycle. By giving a hormonal environment that is similar to the luteal phase of the menstrual cycle, these hormones suppress the surge of luteinizing hormone (LH) that triggers ovulation. After 21 days of continuous use, the body adapts to the hormonal environment, and the hormonal surge that would normally trigger ovulation doesn't occur, resulting in effective contraception.

**Why Each Wrong Option is Incorrect:**

A. **Incorrect answer.** Continuous use of OCPs for 21 days is necessary to suppress the LH surge and prevent ovulation.

B. **Incorrect answer.** While OCPs can be effective, a woman does not need to take them for 28 days to prevent ovulation as this answer suggests.

C. **Incorrect answer.** Taking OCPs for 28 days is not specifically mentioned as the minimum duration required for preventing ovulation.

D. **Incorrect answer.** While OCPs can be effective, the duration required to prevent ovulation is not explicitly stated as 35 days in this answer.

**Clinical Pearl:**

In clinical practice, it is essential for patients to understand the correct duration of hormonal OCPs use for effective contraception. Misinformation could lead to unprotected sexual intercourse during the fertile phase and potential pregnancy.

**Correct Answer:** 21 days.