**Core Concept**
The General Fertility Rate (GFR) is a demographic measure that estimates the number of births per 1,000 women of reproductive age (usually 15-44 years) within a given time period. It is an essential indicator of fertility trends and reproductive health.

**Why the Correct Answer is Right**
The denominator in the General Fertility Rate calculation is crucial, as it represents the population at risk of becoming pregnant. The correct denominator is the number of women of reproductive age (15-44 years) in the population, as this age group is generally fertile and capable of becoming pregnant. The use of this specific age range is essential to accurately estimate fertility rates.

**Why Each Wrong Option is Incorrect**
**Option A:** The use of the total population (men and women of all ages) as the denominator would inflate the GFR and provide an inaccurate representation of fertility trends.
**Option B:** Focusing solely on women of childbearing age (20-39 years) would overlook the reproductive potential of younger women (15-19 years) and older women (40-44 years), potentially leading to underestimation of fertility rates.
**Option C:** Using the number of women of reproductive age in the numerator, rather than the denominator, would result in a measure that is the inverse of the General Fertility Rate.

**Clinical Pearl / High-Yield Fact**
When analyzing demographic data, it is essential to consider the specific age range used as the denominator, as this can significantly impact the accuracy of fertility rate estimates.

**Correct Answer: B. Women of reproductive age (15-44 years)**