Choice "C" is correct. A natural monopoly exists when economic and technical conditions permit only one efficient supplier.
Choice "d" is incorrect. Owning natural resources, even if they are unique, would not create a monopoly. Substitutes for the resource may be available.
Choice "a" is incorrect. Government control may create a monopoly, but not a natural monopoly. This is a regulated monopoly.
Choice "b" is incorrect. Barriers to entry help create a monopoly, but the product must be unique.
