Natural monopolies occur when one producer

A. can meet the market's entire demand.

B. controls the method of production.

C. is the only one authorized to produce a given product.

D. creates unique products.

A natural monopoly occurs when the party that has the monopoly has an advantage over the other parties because it already has the infrastructure. For example, the national post already has the organisation to send around letters, so it might have an advantage over another start-up that would have to first organise it, and charge more for the first sent letters.

The best answer is therefore B, but it's important to note that they control it in the way that they were the first to create it, and other companies could also organise the new means of production, but it would come at a great cost to them.