In Ohio in 1872, the Cleveland Massacre was

a violent labor dispute at Standard Oil refineries in Cleveland.

a takeover by Standard Oil of the refineries in Cleveland.

an attempt to stop Standard Oil from becoming a monopoly.

a failed attempt by Standard Oil to take over other refineries.

The cleveland massacre was initiated by Rockefeller, and it was the start of his big monopoly over oil companies, he will eventually end up buying 22 out of 26 refineries in the whole Ohio State that would be the start of the empire he eventually had.

The answer is: a takeover by Standard Oil of the refineries in Cleveland.

In Ohio in 1872, the Cleveland Massacre was a takeover by Standard Oil of the refineries in Cleveland.