The correct answer is D. They mapped trails and helped build forts
Step-by-step explanation
Mountain men are a group of people who lived in the western mountains of the United States. These mountains were characterized by being difficult to access, so there were no roads in this area, and it did not have many human settlements. Once the idea of building and connecting the entire country was planted, the help of men who inhabited this area was needed to trace the roads and build them because they knew how to move and work on that terrain due to their extensive experience in that terrain. Therefore, the correct answer is D. They mapped trails and helped build forts