**Core Concept**
The dependency ratio is a demographic measure used to assess the number of individuals who are dependent on the working-age population, typically defined as those between 15 and 64 years old. It is calculated by dividing the number of dependents (children and elderly) by the number of working-age individuals.

**Why the Correct Answer is Right**
The correct answer includes individuals less than 15 years old, which represents the dependent population, specifically children. This age group is considered dependent because they are not yet economically productive and rely on others for support. The dependency ratio helps policymakers understand the burden on the working-age population and plan for social security, education, and healthcare needs.

**Why Each Wrong Option is Incorrect**
**Option B:** The elderly population, defined as those 65 years and older, is also a dependent group, but it is not included in the standard calculation of the dependency ratio. The elderly may receive support from family, government, or social security programs.

**Option C:** The age range 30-50 years is considered the working-age population, not the dependent population. This group is typically economically productive and contributes to the workforce.

**Option D:** This option is incorrect because the correct answer is indeed "less than 15 years," which is a component of the dependency ratio.

**Clinical Pearl / High-Yield Fact**
The dependency ratio is a crucial indicator of a country's demographic and economic health. A high dependency ratio can lead to increased social security costs, reduced economic growth, and increased burden on the working-age population.

**✓ Correct Answer: A. Less than 15 years**