The correct answer is - d. Natural resources were depleted from misuse by white settlers.
The white settlers during the westward expansion indeed tried to use a lot of natural resources in order to become rich, but they did not depleted them and left the region poor in natural resources.
The fact that even nowadays, this region is still very rich with natural resources tells us that they were not misused and depleted in the early stages of the settlement by the white people. Also, big portion of the people moving there were obsessed with the gold, so they were focusing only on it as it was something that could have made them rich in no time. The region though is not very rich in gold, and plus most of the gold that was found was on the surface and in the rivers, so there wasn't some big damage initially on the natural resources.
