A lack of political parties would likely result in

consequences of weak political parties. increases chance of political outsiders, changes in national parties weakens parties further. Temporary party organizations. gatherings of ordinary party members (primaries, caucuses, conventions)

Explanation:

c. reserves power to the states and people

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."