John d. rockefeller monopolized the oil-refining business by

The correct answer is: creating trusts.

Rockefeller´s Standard Oil company bought up his competitors and created the US’s first monopoly. Combining many corporations into a trust meant that holders of stock from oil companies handed over their shares to Rockefeller and his associates as a board of trustees, getting in return ‘trust’ certificates which paid dividends without any power.

yup! you are correct it is b.