Final answer:
A Special purpose map is used to display the population density of a specific area, different from Mercator, Physical, or Topographic maps which serve other purposes.
Step-by-step explanation:
The type of map that would show the population density of a specific region is a Special purpose map. This type of map is designed to communicate specific information about particular areas, such as the number of people living per unit of area. Although a Mercator map is useful for navigation, and a Physical map shows natural landscapes, and a Topographic map illustrates elevation and landforms with contour lines, they are not primarily concerned with demographic data like population density.