Answer:
The correct answer is: legal barriers.
Explanation:
A monopoly is a market structure where there is only a single firm, there is a restriction on the entry of firms. This gives firms a certain degree of market power. Â
The monopolies are able to retain their market power through restrictions on the entry of other potential firms. These restrictions are of different types such as exclusive ownership of a resource, legal barriers, increasing returns to scale. Â
In this particular scenario of patents, the barrier to entry is a legal barrier. The other potential firms are legally restricted to enter the market as they do not hold a patent.