**Core Concept**
The dependency ratio is a demographic measure used to describe the proportion of a population that is either too young or too old to work, relative to the working-age population. It is calculated by dividing the sum of the population under 15 years and the population above 65 years by the working-age population (15-64 years).

**Why the Correct Answer is Right**
The dependency ratio is a key indicator of the economic burden of supporting non-working individuals in a population. The working-age population bears the responsibility of supporting the younger and older populations, and a high dependency ratio can lead to increased economic strain. This ratio is often used by policymakers to plan for social security, education, and healthcare systems.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the dependency ratio specifically excludes the working-age population. It focuses on the proportion of individuals who are not contributing to the workforce.
**Option B:** This option is incorrect because the dependency ratio does not include individuals with disabilities or other non-working populations. It only considers children under 15 and individuals above 65 years.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the dependency ratio is a dynamic indicator that changes over time due to population growth, aging, and changes in fertility rates. Policymakers must consider these factors when planning for the future needs of their population.

**Correct Answer: D. Dependency ratio includes the population under 15 years and above 65 years.