**Core Concept**
The Pearl index is a measure used to express the effectiveness of a contraceptive method, specifically its failure rate per 100 woman-years of exposure. It takes into account the number of pregnancies that occur within a given population over a specific period. This index is widely used in reproductive health research and clinical practice to evaluate the efficacy of various contraceptive methods.

**Why the Correct Answer is Right**
The Pearl index is calculated by dividing the number of pregnancies that occur among users of a contraceptive method by the total number of woman-years of exposure. The resulting rate is then multiplied by 100 to express it as a failure rate per 100 woman-years. This allows for a standardized comparison of different contraceptive methods. The Pearl index is a widely accepted and reliable measure of contraceptive efficacy, making **Option B: Per 100 woman years** the correct answer.

**Why Each Wrong Option is Incorrect**
**Option A: Per 10 woman years** is incorrect because it represents a much shorter exposure period, making the calculated failure rate overly optimistic and not representative of real-world use.
**Option C: Per 1000 woman years** and **Option D: Per 10,000 woman years** are incorrect because they represent much longer exposure periods than what is typically used to calculate the Pearl index, making the calculated failure rate overly pessimistic and not practical for clinical use.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the Pearl index is based on the assumption that the contraceptive method is used perfectly, without any failures or errors. In reality, the effectiveness of a contraceptive method may be lower due to user errors, method failures, or other factors. Therefore, it's crucial to consider the Pearl index as a theoretical estimate of efficacy rather than a guarantee of effectiveness.

✓ Correct Answer: B. Per 100 woman years