Criminologist studied differential treatment given to white upper-class boys and white lower-class boys in his classic 1973 study entitled, the saints and roughnecks.

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."

she recieved a treatment not thought to be specifically effective in treating her disorder.