Viewed in historical terms, federalism has been a hostile and dynamic framework that has adjusted to the necessities of the time.
d. i assume the underlined word is "role." reformat if it isn't.
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."