What defines a megacity?

A megacity is specifically defined as a city that has a population of more than 10 million people. This definition highlights the immense scale and density of urban life in such locations, which can significantly impact social, economic, and environmental dynamics. Megacities are often characterized by their vast urban area, substantial economic activity, and complex infrastructure needs to support their large populations.

While other options mention varying population thresholds or features such as urban sprawl, they do not meet the established criteria for categorization as a megacity. Urban sprawl, for instance, refers to the spread of development over a large area which can occur in cities of various sizes. Thus, the size of the population remains the key defining factor for a megacity.