Rockefeller’s great-grandson gives him credit for being such a good businessman. for what does his great-grandson not give him credit?

He didn't have to be as harsh as he was with people.

He could have had a more tender and kind heart.

John Davison Rockefeller was a US investor and entrepreneur. Rockefeller revolutionized the oil sector. In 1870, he founded Standard Oil Company and aggressively commanded it until his official retirement in 1897.

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."