Concept:Population density is the average number of people per unit area; cold, harsh regions generally have very few people.Explanation:The Eurasian Tundra is an extremely cold, largely barren region with permafrost and very limited agriculture.Such conditions cannot support large populations.In contrast, North-West Europe, the Nile Valley, and North-East USA are more favourable for settlement and have higher population densities.Therefore, the Eurasian Tundra has the least population density.Answer:B. Eurasian Tundra