Final answer:
The scenario best exemplifying an oligopoly is when two to four firms produce 70% to 80% of the market's output, which aligns with the definition of an oligopoly as a market dominated by a few firms with significant market power.
Step-by-step explanation:
An oligopoly is a market structure characterized by a small number of firms that dominate the market, often accounting for 70-80% of the output. In the scenario Two to four firms producing 70% to 80% of the output, we observe conditions closely aligned with the definition of an oligopoly. This is because we have a small number of firms that possess a large market share, sufficient enough to influence market prices and outcomes.