D. As Americans moved farther west, they also moved away from their European roots, and a unique American identity began to develop.
Step-by-step explanation:
In the words of Turner, as the people moved towards the west side where they enjoyed great lifestyle and had a comfortable living, they began to have a unique identity of themselves being Americans.
Such comfortable living in the west, made more people from the east attract them towards the west so that they can also enjoy a better life style.
They also had the opportunity to buy land if they move towards the west side of the United States away from Europe.