Concept:Population density shows how crowded a region is by relating population to land area.Explanation:Population density is the average number of people living in a unit area.This unit area is usually expressed in square kilometres or square miles.For example, if 500 people live in 1km2, the population density is 500 people per square kilometre.Thus, the correct definition is the number of people living in a unit area.Answer:B. number of people living in a unit area