**Core Concept**
Progesterone plays a crucial role in emergency contraception by preventing or delaying ovulation, thereby reducing the likelihood of pregnancy. The progesterone used in emergency contraception acts by thickening the cervical mucus, making it difficult for sperm to reach the egg.

**Why the Correct Answer is Right**
The progesterone of choice in emergency contraception is **ulipristal acetate**. It is a selective progesterone receptor modulator (SPRM) that acts by binding to the progesterone receptor, thereby preventing the normal progesterone-mediated changes in the endometrium and cervix that facilitate fertilization. This results in a delay in ovulation and a thickening of the cervical mucus, making it difficult for sperm to reach the egg. Ulipristal acetate is effective in preventing pregnancy when taken within 120 hours (5 days) after unprotected sex.

**Why Each Wrong Option is Incorrect**
**Option A:** Levonorgestrel is a progestin used in emergency contraception, but it is not the most effective option. It is available over-the-counter and is effective when taken within 72 hours (3 days) after unprotected sex.

**Option B:** Norethisterone is a progestin used in some hormonal contraceptives, but it is not the preferred option for emergency contraception.

**Option C:** Medroxyprogesterone acetate is a progestin used in some contraceptive injectables, but it is not the preferred option for emergency contraception.

**Clinical Pearl / High-Yield Fact**
Remember that emergency contraception is most effective when taken within 24 hours of unprotected sex, with effectiveness decreasing as time passes.

**Correct Answer:** D. Ulipristal acetate