Professor swenson is interested in university students' reactions to the death of a popular rock star. for two weeks, professor swenson spends one hour a day in a popular cafeteria, inconspicuously listening to students, joining in their conversations when the topic of the dead rock star comes up, and recording what the students have to say. professor swenson is conducting research.

send them to therapy or somthing like that

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."