Population density is the number of individuals present per unit area or
volume at a given time. If the total number of individuals is
represented by letter N and the number of units of space (area for land
organisms and volume for water organisms) by letter S, then the
population density D can be represented as D =N/S. For instance, number
of animals per square kilometer, number of trees per acre in a forest,
etc. So, the correct answer is 'Population density'.