Concept:Population density shows how crowded a place is by counting people per unit area, like per square kilometer.
Explanation:It is calculated by dividing total population by total land area.
For example, Bihar has about 1102 persons per sq km, while Arunachal Pradesh has only 17 persons per sq km.
Option B correctly defines it as the number of people per unit area of the earth's surface.
Option A describes population distribution (spread of people), not density.
Option C gives the total number of people in an area without referring to the unit area.
Option D refers to the population counted at a specific time (census), not density.
Therefore, only option B matches the correct definition.
Answer:B. the number of people living in a unit area of the earth's surface