Question

read this paragraph.
rocky mountain national park, nicknamed
ocky,\ is a true marvel of the american west. visit this place set aside for its natural beauty and you will encounter a variety of animals, plant life, and even climates. there are four ecosystems inside rocky mountain national park: riparian, montane, subalpine, and alpine tundra. you might see a moose or mule deer drinking from a pond in a riparian ecosystem, which are areas that are near streams or lakes. riparian ecosystems can be found within the other three ecosystems. the montane ecosystem is the lowest in elevation at rocky. there you will find ponderosa pine trees and wide-open mountain meadows, lush with wildflowers in the summer. climb a little higher into the subalpine ecosystem, where long winters and short, cool summers make up the climate. look high into a limber pine tree and you might spot a clarks nutcracker, nibbling on pine seeds. finally, the highest elevation in rocky is the alpine tundra. bitter cold and wind - blasted defines this ecosystem with its jagged mountain tops that are capped with snow even in late august. each of these ecosystems has a quality all its own, but they are all interconnected, comprising a true american treasure:
ocky.\
which of these best states the main idea?
there are four ecosystems inside rocky mountain national park: riparian, montane, subalpine, and alpine tundra.
rocky mountain national park, nicknamed
ocky,\ is a true marvel of the american west.
the riparian ecosystem can be found within the other three ecosystems.
each of these ecosystems has a quality all its own, but they are all interconnected, comprising a true american treasure:
ocky.\

Explanation:

The paragraph begins by introducing Rocky Mountain National Park as a "true marvel of the American West" and then goes on to describe its ecosystems. The other options are details (like the number of ecosystems, a specific ecosystem's location, or a characteristic of ecosystems) that support the main idea. The main idea is the overarching statement about the park being a marvel, which the details about the ecosystems illustrate.

Answer:

Rocky Mountain National Park, nicknamed "Rocky," is a true marvel of the American West.