**Core Concept**
NORPLANT, also known as levonorgestrel implant, is a long-acting reversible contraceptive method that releases a steady dose of the progestin hormone levonorgestrel. This hormone is responsible for the contraceptive effect through its action on the reproductive system.

**Why the Correct Answer is Right**
The levonorgestrel implant is designed to release a constant level of levonorgestrel into the bloodstream, which suppresses ovulation by inhibiting the LH surge. This action prevents the release of an egg from the ovary, effectively preventing fertilization. The implant also thickens cervical mucus, making it difficult for sperm to reach the egg, and may alter the endometrium, making it less receptive to implantation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the duration of NORPLANT's effectiveness.
**Option B:** This option is not accurate, as NORPLANT's effectiveness does not depend on user compliance.
**Option C:** This option is not correct, as NORPLANT's effectiveness is not limited by its duration of use.

**Clinical Pearl / High-Yield Fact**
It's essential to note that NORPLANT's effectiveness is not affected by weight or body mass index (BMI), making it a suitable option for women with a wide range of body types.

**Correct Answer: C. 5 years**