Imagine you are part of a design team tasked with creating an industrial machine for the mass production of peanut butter and jelly sandwiches. consider that you have successfully completed the design tasks and created a prototype. looking back on what it took to be successful:

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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."