Final answer:
A researcher examining population size, age structure, urbanization, and ethnic composition would use demographics as the basis for their study.
Step-by-step explanation:
If a researcher were examining population size, age structure, urbanization degree, and ethnic composition, they would likely be using demographics as their base. Demographics include statistical data related to the population such as the aspects mentioned in the question: size, age structure, urbanization, and ethnicity. The field of demography studies these characteristics and how they change over time, accounting for birth rates, death rates, and life expectancy, among other facets of population study. Researchers can analyze, interpret, and make predictions based on these demographic factors.