**Core Concept**
The Pearl index is a statistical term used to measure the effectiveness of contraceptive methods by calculating the number of unintended pregnancies per unit of exposure. It's a widely used metric in reproductive health and family planning.

**Why the Correct Answer is Right**
The Pearl index is defined as the number of accidental pregnancies per 100 woman-years of exposure. A woman-year is a unit of measurement representing one woman exposed to a contraceptive method for one year. This calculation allows for a standardized comparison of contraceptive efficacy across different populations and studies. The Pearl index takes into account the duration of exposure, making it a more accurate measure of contraceptive effectiveness.

**Why Each Wrong Option is Incorrect**
**Option A:** The Pearl index does not measure accidental pregnancies per 1000 population. This option is incorrect because the Pearl index focuses on the reproductive outcome (pregnancy) rather than the overall population size.
**Option B:** The Pearl index is not defined as the number of accidental pregnancies per 100 live births. This option is incorrect because the Pearl index specifically measures contraceptive effectiveness, not fertility rates or birth rates.
**Option C:** The Pearl index does not specifically focus on women in the age group of 15 to 44 years. While this age group may be relevant for reproductive health studies, the Pearl index is applicable to women of all ages who are using contraceptive methods.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Pearl index is a specific metric for measuring contraceptive effectiveness, and it's not interchangeable with other terms like "pregnancy rate" or "fertility rate." Familiarity with the Pearl index will help you critically evaluate the efficacy of contraceptive methods in various studies and clinical settings.

**✓ Correct Answer: D. 100 women years. 100 woman-years of exposure.**