An industry that contains a firm that is the only producer of a good or service for which there are no close substitutes and for which entry by potential rivals is prohibitively difficult is: a. a duopoly. b. a monopoly. c. an oligopoly. d. perfect competition.

B. a monopoly

Explanation:

In the given situation, the most appropriate option is a monopoly, as it is that market in which there is only a single seller who sells the unique product to the end consumers which brings no competition in the market.  

The price of the seller is high as he is only the seller of the specific product. In this, the competition is difficult as other sellers have no close substitutes for the product.  

Hence, option b is correct.

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Explanation: