Population density refers to:

The correct option is Persons per square kilometre.

Explanation:

Population density is a measurement of population per unit area. It is most commonly calculated for human populations and expressed as the number of persons living per unit of area, typically per square kilometre (or square mile).

The formula for calculating population density is:

$\text{Population Density}=\frac{\text{Total Population}}{\text{Land Area (in sq. km)}}$

Therefore, it specifically measures the concentration of individuals (persons) within a standard spatial unit of measurement, which is Persons per square kilometre.