Luke had been using a drug that produced altered and false visual and auditory perceptions, but he easily stopped using it after a few months of experimentation. He was MOST likely using:

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."

you answered your own : -/

answer; ///any two strangers are likely to share many coincidental similarities;