The correct answer is - an urban area.
The urban areas have the largest populations, and they are also by far the most densely populated areas. Unlike the suburban areas that are medium sized, and where the people mostly live in houses, or in the rural areas where the people live in houses that further away from one another, the people in the urban areas tend to live mostly in buildings. The buildings are very high, and they can accommodate thousands of people in them, resulting in a very high density of the population. The urban areas are also the most developed, they are the biggest settlements, and the people tend to move toward them in order to get better life and opportunities, resulting in an ever growing urban population.