About half of all federal discretionary spending goes towards
a. social security.
b. civilian salaries.
c. education.
d. defense.

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."

i only have part a but here is the first one: - lewis and clark are born

- lewis marries sacajawea and clark is jealous  

- clark drowns lewis and then goes to jail

answer; conditional stimulus will trigger a conditioned response;