**Core Concept**
The Pearl Index is a widely used measure to evaluate the effectiveness of a contraceptive method. It represents the number of pregnancies per 100 woman-years of exposure. This calculation takes into account the duration of use and the number of women exposed to the contraceptive method.

**Why the Correct Answer is Right**
To calculate the Pearl Index, we use the formula: (Number of pregnancies / Number of women) x 100 x Duration of exposure (in years). In this case, 10 women became pregnant out of 100 over 24 months, which is equivalent to 2 years. Therefore, the Pearl Index is calculated as: (10 / 100) x 100 x 2 = 20.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it doesn't take into account the duration of exposure. A Pearl Index of 10 would be correct if the women were exposed for only 1 year, not 2 years.

**Option B:** This option is incorrect because it incorrectly assumes that the number of pregnancies is the denominator. The correct denominator is the number of women, which is 100 in this case.

**Option C:** This option is incorrect because it incorrectly assumes that the duration of exposure is 1 year, not 2 years.

**Option D:** This option is incorrect because it doesn't take into account the number of women who became pregnant.

**Clinical Pearl / High-Yield Fact**
The Pearl Index is a useful measure to evaluate the effectiveness of a contraceptive method, but it has its limitations. For example, it doesn't take into account the age of the women, their fertility, or the type of contraceptive used.

**Correct Answer: A. 20.**