Final answer:
The world's most densely populated urban centers are located along coasts, natural harbors, and river valleys, which offer more resources and are conducive to trade, living, and economic activities, unlike arid plains, polar regions, and tundra biomes that are less populated due to harsher conditions. The correct option is (a).
Step-by-step explanation:
The world's most densely populated urban centers are located along coasts, natural harbors, and river valleys. This pattern is illustrated by the large cities of Australia being primarily on the coast, and the majority of North America's, Latin America's, and Europe's populations residing in urban areas.
The areas with the highest human population density, such as South and Southeast Asia, the Middle East, and Central America, also coincide with significant habitat destruction, impacting both terrestrial habitats and coastal ecosystems like coral reefs.
In contrast, arid plains, polar regions, and tundra biomes, due to their challenging living conditions and lack of resources such as water, are less suited to support large human populations and thus do not have densely populated urban centers.