What is a market that runs most efficiently when one large firm supplies all of the output referred to as?

What is a market that runs most efficiently when one large firm supplies all of the output referred to as? Natural monopoly. A natural monopoly happens when there are high fixed costs or start-up costs when running a business in certain industries. A natural monopoly example are pipelines that run for water and gas. This is because they are expensive to start and run but also extremly necessary and specific to the industry.

The answer to this question is Natural monopoly.