Final answer:
A monopoly arises when a single firm dominates the market sales with a product lacking close substitutes, exemplified by Microsoft in the operating systems market.
Step-by-step explanation:
A monopoly occurs when a single firm has the bulk of sales in a specific market. This is the scenario where a single company sells a product for which there are no close substitutes, such as Microsoft's dominance in the operating systems market.
A market is often defined by the goods and services sold within it, and a monopoly essentially controls this market by being the primary provider for these goods and services. An example of a monopoly is Microsoft, which dominates the operating systems market and has been considered a monopoly due to its control over this specific market.