Which group (pueblos, navajos, sioux) was most impacted by the transcontinental railroads? give two reasons why.

I believe that the Sioux were most impacted by the Transcontinental railroads. The building of the railroads impacted the Great Plains tribes, and among your options, only the Sioux were situated there, whereas Pueblos and Navajos were elsewhere.
The reasons why they were most impacted are numerous. First of all, when the railroads were built, they passed through the Native American lands, which were protected by the American government which obviously went back on their word. Additionally, when the railroads were made, it affected the flora and fauna in the Great Plains, and many animals (especially the bison) were almost extinct which affected the Sioux, who were a hunting tribe.