What is the study of human populations, including size and distribution?

Demography is the correct answer because it specifically focuses on the statistical study of human populations. This includes analyzing aspects such as population size, distribution, density, age structure, and changes over time due to factors like birth rates, death rates, and migration. Demographers gather and analyze data to understand how populations evolve and can influence policy and planning in areas like urban development, healthcare, and education.

In contrast, anthropology is the study of human societies, cultures, and their development, while geography encompasses a broader range of topics that can include physical landscapes, human-environment interactions, and spatial analysis. Population ecology deals more with the interactions between populations and their environments, focusing on the ecological factors that affect population dynamics rather than the statistical characteristics of human populations directly.