What is a market that runs most efficiently when one large firm supplies all of the output referred to as? a government monopoly
a natural monopoly
a franchise
market power
(Gradpoint)

a natural monopoly

Explanation:

A monopoly is a market structure which is typically characterized by a single-seller (one seller) who sells a unique product in the market by dominance. This ultimately implies that, it is a market structure wherein the seller has no competitor because he is solely responsible for the sale of unique products without close substitutes.

A monopolist refers to any individual that deals with the sales of unique products in a monopolistic market.

On a related note, a natural monopoly is a market that runs most efficiently when all of the output is supplied by one large business firm. Thus, a business firm is considered to be a natural monopoly if it's capable of producing the total output of the market at a lower cost than two or more business firms could.

Some examples of natural monopoly are the United States Postal Service, electricity grid, water supply, gas network, sewer services, energy distributors, railway service, etc.