World systems theory focuses on

A) internal factors in individual countries.
B) patterns of migration and other population issues.
C) the belief that there is a world economic system that must be understood as a single unit.
D) neocolonial relationships between individual countries and their former colonizers.

World systems theory is a social, economic, and political analysis that suggests there is a world economic system that must be understood as a single unit. This theory focuses on how different countries and regions of the world are integrated into a global economic system, with some countries benefiting more than others. It sees the world as divided into core countries (wealthy and dominant), semi-peripheral countries (partially industrialized and industrializing), and peripheral countries (often dependent on core countries for resources and labor). Therefore, the best choice is option C.