The process by which urban populations and their activities become more complex and heterogeneous over time is called:

A) human ecology
B) differentiation
C) concentric zone theory
D) differential organization
E) zoning

The process of urban populations becoming more complex and heterogeneous is known as differentiation. This process is characterized by the development of specialized economic, social, and cultural activities that contribute to the growth and diversification of the urban area. Differentiation occurs over time as new groups and sectors emerge and interact with one another, resulting in an increasingly complex and heterogeneous urban landscape. The other options are not directly related to this process - Human ecology focuses on the relationships between humans and their natural and built environments, Concentric zone theory is a model of urban growth, Differential organization refers to the way that criminal activities are organized in urban areas, and Zoning refers to the regulation of land use in urban areas.