Natural monopolies occur when one producer
a. can meet markets entire demand
b. controls the method of production
c. is the only one authorized to produce a given product
d. created unique products

d. created unique products is your answer

Natural monopolies occur because the start-up cost is high (with high chances of not succeeding). Example can be scarcity of material needed to use, technological usage and advancement for the industry is patented and cannot be widely used, and the product being produced is more efficient if there is only one company in the area producing the product (such as electricity to homes, water, etc.)

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answer; (c) run the name by everyone you can;