**Core Concept:** Norplant is a long-acting reversible contraceptive method containing levonorgestrel, a progestin hormone. Sub-dermal implants release hormones continuously into the bloodstream, preventing fertilization by disrupting the menstrual cycle and suppressing ovulation.

**Why the Correct Answer is Right:** The correct answer is B (ethinylestradiol) because ethinylestradiol is an estrogen hormone that, when combined with levonorgestrel, forms the active ingredient in Norplant contraceptive implant. This combination of hormones helps in suppressing ovulation, thickening cervical mucus, and thinning the uterine lining, ultimately preventing fertilization.

**Why Each Wrong Option is Incorrect:**

A. Oral contraceptives (ethinylestradiol and levonorgestrel) are administered orally, while the correct answer describes a sub-dermal implant, which is the correct form of Norplant.

C. Ovulation inhibitors like clomiphene citrate are oral medications prescribed for polycystic ovary syndrome (PCOS) patients, not sub-dermal implants like Norplant.

D. Progesterone-only contraceptives primarily thicken cervical mucus, while Norplant is a combination of estrogen and progestin (levonorgestrel) for more comprehensive contraception.

**Clinical Pearl:** Norplant is a highly effective contraceptive method with an extremely low failure rate when used correctly and consistently. It offers long-acting, reversible contraception with minimal patient compliance requirements, making it an essential tool in family planning strategies.

**Correct Answer:** C. Progesterone-only (levonorgestrel)