The correct answer is: D. Oligopoly and monopoly.
A barrier to entry is an obstacle that makes it difficult for new firms to enter a market. Barriers to entry can be natural, such as economies of scale, or they can be created by the government, such as patents or licenses.
Oligopoly is a market structure in which a small number of firms dominate the market. This can lead to high barriers to entry, as new firms may find it difficult to compete with the established firms.
Monopoly is a market structure in which there is only one firm. This is the highest form of barrier to entry, as there is no competition from other firms.
Perfect competition is a market structure in which there are many firms, all of which produce identical products. This results in low barriers to entry, as new firms can easily enter the market and compete with the existing firms.
Differentiated competition is a market structure in which there are many firms, all of which produce products that are similar but not identical. This results in moderate barriers to entry, as new firms can enter the market and compete with the existing firms, but they may find it difficult to compete on price alone.
In conclusion, the two types of markets with the highest barriers to entry are oligopoly and monopoly.