At jack's office a number of men start wearing hawaiian shirts on fridays. soon jack begins wearing a hawaiian shirt on fridays. jack's behavior is an example of :

using the scientific method.

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."