Which organization has line authority to oversee the management of multiple incidents being handled by separate incident command organizations?
a. joint command
b. united command
c. multiagency command
d. area command

answer; (true);

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."