A natural monopoly refers to a monopoly that is defended from direct competition by. a. economies of scale over a broad range of output
b. a government franchise
c. control over a vital input
d. a patent or copyright

a. economies of scale over a broad range of output

Explanation:

Monopoly is a market condition in which there are no competitors and a particular company produces only that type of product and services. This is common in sectors which are very expensive to handle by the government.

However natural monopoly refers to a monopoly that is defended from direct competition by economies of scale over a broad range of output. It usually has the strong backing of the government.