**Core Concept**
The population growth rate is typically expressed as a percentage, and it can be used to calculate the time required for the population to double. This concept is crucial in demography and epidemiology to understand population trends and make informed decisions.

**Why the Correct Answer is Right**
The rule of 72 is a mathematical formula used to estimate the number of years required for a population to double at a given annual growth rate. The formula is: years to double = 72 / growth rate (%). In this case, the growth rate is 1.5-2%, so we can calculate the number of years required to double the population using this formula. For a growth rate of 1.5%, the years to double would be 72 / 1.5 = 48 years, and for a growth rate of 2%, it would be 72 / 2 = 36 years.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the growth rate of the population. A fixed number of years would not accurately reflect the population growth.
**Option B:** This option is incorrect because it is not related to the population growth rate. The number of years required to double the population depends on the growth rate, not the size of the population.
**Option C:** This option is incorrect because it is a random number and does not follow the rule of 72. It does not take into account the growth rate of the population.

**Clinical Pearl / High-Yield Fact**
The rule of 72 is a simple yet powerful tool to estimate population growth. It can be applied to various fields, including demography, epidemiology, and economics.

**Correct Answer:** D.