Size doesn't make sense because it means how big or small an area is.
Density means how many people there are in a certain area.
Dispersion means distributing people over a wide area.
The answer is Density.
This is also known as the "Population Density." This means the amount of people there are in a unit of area. It shows how many people prefer to live in a certain area compared to area where people don't prefer to live.
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