**Core Concept**
The Pearl Index is a measure of the effectiveness of a contraceptive method, calculated as the number of accidental pregnancies per 100 woman-years of exposure. It is a widely used metric to evaluate the efficacy of contraceptive methods.

**Why the Correct Answer is Right**
To calculate the Pearl Index, the number of accidental pregnancies is divided by the number of woman-years of exposure, and the result is multiplied by 100. In this case, 2.5 accidental pregnancies occurred over an unspecified period. However, to calculate the Pearl Index, we need to assume a specific time frame, typically one year, for the exposures. If we assume the 2.5 accidental pregnancies occurred over 250 woman-years (2.5 pregnancies / 1 pregnancy per year), the Pearl Index would be 1 (2.5 pregnancies / 250 woman-years * 100).

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a specific calculation or formula for the Pearl Index.
**Option B:** This option is incorrect because it would require more information about the duration of exposure to calculate the Pearl Index accurately.
**Option C:** This option is incorrect because it does not provide a valid calculation for the Pearl Index.

**Clinical Pearl / High-Yield Fact**
The Pearl Index is a useful metric for evaluating the effectiveness of contraceptive methods, but it has limitations, such as not accounting for the duration of exposure or the timing of accidental pregnancies.

**Correct Answer:** A. 1