Which of the following best defines a megacity?

A megacity is specifically defined as a city that has a population of over 10 million people. This definition highlights the scale and urbanization complexity of such cities, as they typically face unique challenges associated with their large populations, such as transportation, housing, infrastructure, and providing services like healthcare and education. The concept of a megacity often relates to global trends in urbanization, where more people are moving to urban areas, leading to significant population growth in these sprawling metropolises.

In contrast, the other answers describe cities that may be significant in size but do not meet the stringent criteria for being considered a megacity. For example, a population of over 1 million or even 5 million, while large, does not encapsulate the extensive dynamics and challenges that come with a population exceeding 10 million, which is relevant to megacities. Additionally, defining a city solely by its status as a national capital does not take population size into account, as there are many capital cities that do not fall into the megacity category.