A monopoly is a market structure, in which

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.