Density of population means number of persons per:

The correct answer is B. Square metre.

Density of population is the number of people per unit area. It is usually expressed as people per square kilometre or people per square mile. The density of population can be used to compare the population of different countries or regions. It can also be used to track changes in population over time.

The other options are incorrect because they are not units of area. A square foot is a unit of area in the imperial and US customary systems of measurement. A square inch is a unit of area in the imperial and US customary systems of measurement. A square kilometre is a unit of area in the metric system.