Because the French and Indian War was so costly, the King and Parliament passed the Proclamation of 1763 in order to: A. force Native Americans to move further west.
B. make allies with Native American tribes in the Appalachian Mountains.
C. prevent British settlers from moving post the Appalachian Mountains.
D. force colonists to pay for the French and Indian War.

the answer your looking for is c. : )

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."