Answer: A) control of resources
Step-by-step explanation:
One of the ways that Monopolies create barriers to entry is by controlling the natural resource. This question is a an example of that happening.
By controlling the natural resource, the monopoly can decide who it wants to sell to and at what price. It could even decide not to sell the resource at all which would make it the sole producer of the goods derived from the resource which would effectively force every competitor out of the market as they would be unable to produce anything.