Final answer:
The highest barrier to entry is found in pure monopolies, due to factors like economies of scale and legal restrictions. The lowest barrier to entry is present in perfectly competitive markets, where there are no barriers to entry or exit.
Step-by-step explanation:
The student's question pertains to the level of barriers to entry in different market structures. The highest barrier to entry can be found with pure monopolies, where legal, technological, or market forces prevent potential competitors from entering a market. This could be due to natural monopolies, where economies of scale make it impractical for new entrants to compete, or legal monopolies, where laws explicitly restrict competition.
On the other hand, the lower barrier to entry can be found with pure competition, in perfectly competitive markets which have no barriers to entry or exit. Here, firms can enter and leave the market with ease, leading to a high level of competition and little to no economic profit in the long run.