How can we guarantee the training of human beings for decent productive work, for individual and social benefit?

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."

answer; cindy crawford tina feycarrie underwood///meryl streep;

answer; motor learning;

answer; rational if melanie values sleep more highly than the benefits she expects to receive from attending the class;