The correct answer is "A".
The Oregon Trail was one of the first routes that opened the west to the many immigrants that would come from the east in the years to come. It connected the Missouri river with the valleys of what is now the state of Oregon, crossing the states of Kansas, Nebraska, and Wyoming.
Due to the harsh weather conditions that could appear in the mountains during the winter, the pioneers preferred to avoid crossing this part of the trail during winter, as unexpected weather phenomena could bring problems to the expeditions.