**Core Concept**
The dependency ratio is a demographic measure used to calculate the number of dependents (non-working population) per 100 working-age individuals in a population. It is calculated by dividing the sum of the population below 15 years and above 65 years by the population between 15 and 64 years.

**Why the Correct Answer is Right**
To calculate the dependency ratio, we need to first determine the population between 15 and 64 years. Given that 20% of the population is below 15 years and 15% is above 65 years, the remaining 65% is between 15 and 64 years. The formula for the dependency ratio is: (Population below 15 + Population above 65) / Population between 15 and 64. Plugging in the values, we get: (20 + 15) / 65 = 35 / 65 = 0.538 or 53.8.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we'll move to the next one.

**Option B:** This is not a valid option as the question does not provide any calculations for this option.

**Option C:** This option is not provided, so we'll move to the next one.

**Option D:** This is not a valid option as the question does not provide any calculations for this option.

**Clinical Pearl / High-Yield Fact**
The dependency ratio can be used to estimate the burden on the working-age population and plan for social security, healthcare, and education.

**Correct Answer: D. 53.8.**