## Core Concept
The Pearl index, also known as the Pearl rate, is a commonly used measure to evaluate the effectiveness of a contraceptive method. It represents the number of failures (accidental pregnancies) per 100 woman-years of exposure or usage.

## Why the Correct Answer is Right
The Pearl index is calculated as the number of accidental pregnancies divided by the number of women using the contraceptive method multiplied by the duration of use (usually expressed in years), then multiplied by 100 to express it as a rate per 100 woman-years. This measure helps in comparing the efficacy of different contraceptive methods. A lower Pearl index indicates a more effective contraceptive method.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incomplete and does not provide a specific measure or unit, making it impossible to evaluate its correctness.
- **Option B:** This option suggests 1000 woman-years, which is not the standard unit used for calculating the Pearl index.
- **Option C:** This option suggests 100 women-months, which, although close, does not accurately represent the standard calculation of the Pearl index that uses woman-years.
- **Option D:** This option is not provided, but based on the correct answer, we can infer that it likely corresponds to "100 woman-years."

## Clinical Pearl / High-Yield Fact
A key point to remember is that the Pearl index is a useful tool for comparing the effectiveness of contraceptive methods, but it does not account for factors such as user compliance or the method's impact on various populations. For example, the typical use Pearl index for combined oral contraceptives is around 7-9, whereas the perfect use Pearl index is significantly lower, around 0.1-0.3.

**Correct Answer: B. 100 woman-years.**