**Core Concept**
Dependency ratio is a demographic measure that expresses the ratio of dependents (people who are not in the workforce) to the workforce (people who are capable of working) in a population. The denominator in the dependency ratio represents the workforce population.

**Why the Correct Answer is Right**
The workforce population is typically defined as the population aged 15-64 years, as this age group is generally considered capable of working. This age range is used to calculate the denominator in the dependency ratio because it represents the number of people who are potentially contributing to the workforce and supporting the dependents. The age range of 15-64 years is chosen because it excludes children under 15 years (who are considered dependents) and individuals 65 years and older (who are also considered dependents and may be retired).

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the population aged 0-14 years is considered the dependent population, not the workforce.
**Option B:** This option is incorrect because the population aged 65 years and older is considered the dependent population, not the workforce.
**Option C:** This option is incorrect because the population aged 0-64 years is not a valid definition of the workforce population.

**Clinical Pearl / High-Yield Fact**
When calculating the dependency ratio, it's essential to use the correct age ranges to ensure accurate representation of the workforce and dependent populations. Using the wrong age ranges can lead to inaccurate conclusions about the demographic makeup of a population.

**Correct Answer: D. Population aged 15-64 years.**