The president’s role as chief executive is to

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Explanation:

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."

the chair is the committee member from the majority party who has the most seniority. in the south carolina senate, the chair is the committee member from the majority party who has the most committee seniority.