Question

What made Standard Oil a horizontal integration monopoly?
It controlled all aspects of oil production.
It owned ninety percent of US oil refineries.
It formed a trust.
It operated all across the United States.

Answer 1

It owned ninety percent of US oil refineries.

Reason: horizontal integration means buying out or merging with competitors at the same stage of production (refining). Standard Oil achieved a monopoly by consolidating most U.S. refineries, not by controlling every phase (vertical integration) or just by operating nationwide or forming a trust (the trust was a legal device, not the definition of horizontal integration).