Which one of the following ecosystems is endangered by increasing urbanization?

Estuaries are coastal ecosystems where freshwater from rivers mixes with saltwater from the ocean. These areas are critically endangered by increasing urbanization because cities often expand along coastlines, leading to habitat destruction, pollution from runoff, and altered water flow. Urban development frequently involves dredging, filling wetlands, and constructing infrastructure that directly destroys estuarine habitats. While rivers (Option A) are affected by urbanization, they are not as uniquely vulnerable to the specific pressures of coastal city expansion as estuaries. Islands (Option B) are primarily threatened by sea-level rise and invasive species rather than direct urban sprawl. Mangroves (Option D) are also endangered by urbanization, but estuaries represent the broader category of coastal transition zones that face immediate pressure from port development and city growth. Therefore, Option C is the most accurate answer regarding ecosystems specifically threatened by urban expansion in coastal regions.