What are the major barriers to entry that explain the existence of monopoly?

What will be an ideal response?

There are at least four types of entry barriers that create the conditions for pure monopoly. First, economies of scale means that in some industries large-scale production is necessary to achieve the most efficient level of production. In this case, the long-run average cost curve will tend to decline over a wide range of output for a product. Either a few firms, or in the extreme, one firm, will be necessary to provide market demand in the most efficient way. In the extreme, there are natural monopolies that arise when competition between firms is simply inefficient or impractical. The best examples of natural monopolies are public utilities, which are given the exclusive right by government to provide the utility services. Second, the government can restrict entry into a market by granting patents and licenses. A patent gives exclusive production rights to a firm for twenty years. Licenses limit entry into a business or an occupation. Third, pure monopoly arises when a firm has complete ownership of an essential resource, such as aluminum or copper. Fourth, there are pricing or strategic barriers to entry.