Final answer:
The United States' greatest accomplishment of bringing civilization to the west is a subject of discussion in Social Studies. Frederick Jackson Turner is associated with this accomplishment through his 'Frontier Thesis' that highlighted the importance of westward expansion in shaping American society.
Step-by-step explanation:
The subject of this question is Social Studies.
The statement is discussing the greatest accomplishment of the United States of America so far in terms of bringing civilization to the west. This refers to the colonization and settlement of the western regions of the country, which played a significant role in the expansion and development of the nation.
Out of the given options, the key figure associated with this accomplishment is Frederick Jackson Turner. He was a historian who presented the influential 'Frontier Thesis' in 1893, arguing that the westward expansion and the closing of the frontier shaped American democracy and society.
Learn more about westward expansion in the United States