The correct answer is D.
A monopoly is a market structure in which there is only one firm producing a product or service that has no close substitutes. This means that the firm has a great deal of market power and can control prices and output.
Option A is incorrect because it describes a situation in which a single firm exercises its power over smaller firms. This is not necessarily a monopoly, as the smaller firms may still be able to compete with the larger firm.
Option B is incorrect because it describes a situation in which a single firm produces a product with a wide variety of very close substitutes. This is not necessarily a monopoly, as the firm may still face competition from other firms producing similar products.
Option C is incorrect because it describes a situation in which each firm is run by a small proprietor. This is not necessarily a monopoly, as the firms may still compete with each other.