Why were the mountain men considered invaluable to the united states

the mountain men were the ones who blazed a trail though the applachan and colorado mountain ranges  

The mountain men were explorers most common in the 19th century. They were essential in opening up the trails through the new territories of the far west. They played a key role in the expansion into the west being the first to explore these territories. As mapmakers, land surveyors, treaty negotiators as well as Indian translators, they became invaluable both to stagecoach and railroad companies, government officials and the frontier army.

cherokee and arikara

i believe your answer is d)