Question

what did john d. rockefeller do to establish standard oil as a monopoly?
kept discovering oil
bought stock options
invested in banks
set up a trust

Explanation:

John D. Rockefeller established Standard Oil as a monopoly by creating a trust (the Standard Oil Trust) to consolidate control over the oil industry. The other options: "kept discovering oil" is not how monopolies are established, "bought stock options" and "invested in banks" are not related to his monopoly - building for Standard Oil.

Answer:

D. set up a trust (Note: Assuming the last option is labeled D as per typical multiple - choice formatting with the options in order. If the original had different labels, adjust accordingly. Here the options are presented with the last one being "set up a trust" which is the correct action for Rockefeller's monopoly establishment.)