Final answer:
A monopoly is defined as a firm that produces a product for which there are no close substitutes.
Step-by-step explanation:
A monopoly is defined as a firm that produces a product for which there are no close substitutes. Option 3) that can ignore the actions of all other firms because it produces a product for which there are no close substitutes is the correct definition of a monopoly. For example, Microsoft is considered a monopoly because it dominates the operating systems market, and there are no other operating systems that are direct substitutes for Windows.