In which situation is it necessary to know both population density and population size?

Correct answer: When estimating both service demand and total beneficiaries

The direct answer is A: when estimating both service demand and the total number of beneficiaries. Population size tells us the total number of people, while density tells us how closely those people are spread across an area. For planning a hospital, school, water supply or transport system, authorities need to know how many people may use the service and where pressure is concentrated. Option A is correct because it uses both measures together. Option B is unrelated to population studies; pencil colour needs neither population size nor density. Option C asks for a river’s old name, which is a historical or geographical naming issue. Option D asks for mountain height, which requires elevation measurement, not population information. A large area may have many people but low density, while a small area may have fewer people but high density. Thus, size and density answer different questions and should not be confused. Exam cue: total beneficiaries means population size; local concentration and pressure mean density.