Respuesta :
The correct answer is answer D ("Westward expansion led to loss of life and territory for Native Americans and added to the growing conflict between 'free' and 'slave' states before the Civil War").
A rise in population and economic oppurtunities equally, prompted a massive migration to the vast lands west of Mississippi that the U.S. Federal Government was presenting to be "available". In reality of course, this expansion came at the expense of the lives of the entire Native American population settled there long before. There was no "free land", just a power imbalance that would reach a peak leading to the Civil War.
Hope this cleared things up!