A high population density means that there is a large number of people in a particular area, usually measured as the number of persons per unit area. This means that the population in that area is dense, or crowded. Population density is an important factor in understanding the level of pressure that the population is placing on the available natural resources, the level of urbanization, the level of pollution, and the overall quality of life of the population. Therefore, option B - "persons per unit area" - is the correct answer.