What are the four most important ways a firm becomes a monopoly?

A firm can become a monopoly through exclusive control over a key resource, allowing it to dominate the market. It may achieve significant economies of scale, making it more cost-effective than potential competitors. Legal barriers, such as patents or government regulations, can also grant a firm monopoly power by preventing others from entering the market. Lastly, aggressive business practices, like predatory pricing, can drive competitors out, solidifying the firm's monopolistic position.