The correct answer is C. missionaries and settlers joined the fur traders who went there in earlier years.
Starting in the 1840s, the American settlement in the Oregon Country saw a significant increase as more settlers, including missionaries, moved westward. The Oregon Trail became a popular route, and the influx of these new settlers was in addition to the fur traders who had been in the region before them. This period marked a transition from primarily fur trading to more agricultural settlement and homesteading.