The correct answer is C. A natural monopoly is a market situation in which a single firm can supply the market output more efficiently than many firms. This is because the production of a good or service in a natural monopoly has high fixed costs and low marginal costs. This means that the average cost of production decreases as the firm produces more output. As a result, a single firm can produce the good or service at a lower cost than multiple firms.
Option A is incorrect because it does not specify that the firm must be able to produce the good or service more efficiently than multiple firms. Option B is incorrect because it does not specify that the firm must be the only supplier of the good or service. Option D is incorrect because it is the only option that is not a correct description of a natural monopoly.