You feel obligated to vote in the next election, especially because you are usually the first one to complain about the incumbent president. however, you have not had the time to research the positions taken by the candidates. social psychologists would confidently predict that you are likely to choose the candidate

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."