How did Soviet rule change the Caucasus? A. The Soviets encouraged the people to show pride in their ethnic identities.
B. The region changed from an agricultural area to an urban and industrial area.
C. Islam slowly became the dominant religion throughout the region.
D. The nations in the region finally gained their independence under Soviet rule.

B. The region changed from an agricultural area to an urban and industrial area.

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."