A town has a population of 120,000 and a built-up area of 30 km². What is the population density of the town?

Population density is calculated by dividing the total population by the total area: 120,000 ÷ 30 = 4,000 persons per km². Option A (3,000) results from dividing 90,000 by 30, a common subtraction error. Option C (3,600) may arise from an incorrect area estimate, and Option D (4,500) from dividing 135,000 by 30. The correct calculation yields 4,000 persons per km².