**Core Concept**
Emergency contraception refers to the use of medications or devices to prevent pregnancy after unprotected sex or contraceptive failure. It works by inhibiting or delaying ovulation, fertilization, or implantation of a fertilized egg.

**Why the Correct Answer is Right**
The most commonly used emergency contraceptive pills (ECPs) contain high doses of levonorgestrel, a synthetic progestin. Levonorgestrel works by inhibiting ovulation and preventing fertilization. It may also have some effects on the endometrium, making it less receptive to implantation. ECPs are most effective when taken within 72 hours of unprotected sex, but their efficacy decreases with time.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the active ingredient in emergency contraception. While some ECPs contain ulipristal acetate, it is not the most commonly used or well-known option.

**Option B:** This option is incorrect because it does not mention the primary mechanism of action of emergency contraception. While some ECPs may have some effects on the endometrium, the primary goal is to prevent ovulation and fertilization.

**Option C:** This option is incorrect because it does not specify the type of emergency contraception. While some devices, such as the intrauterine device (IUD), can be used for emergency contraception, they are not the most common or widely used option.

**Clinical Pearl / High-Yield Fact**
It's essential to note that emergency contraception is not a substitute for regular birth control. It should only be used in cases of unprotected sex or contraceptive failure, and not as a regular form of birth control.

**Correct Answer:** C. Levonorgestrel