The correct answer is A) To contrast the city with the much smaller ecosystems that existed before it.
The author most likely emphasizes the size of New York City to contrast the city with the much smaller ecosystems that existed before it.
That is why the author writes: "...what human beings with explosive hardware can do, not crude processes like erosion or rot... Nevertheless, the time it would take nature to rid itself of what urbanity has wrought may be less than we might suspect..."
The author refers that so much damage has done to nature with all those steel and concrete used to build a huge city, in a place where once was threes, plants, greens, and natural habitat. He compares it with previous ecosystems that existed before. That is why the author reflects on the issue of rampant urbanization.