Answer: b. increasing returns to scale.
Explanation:
With the high capital costs having enabled decreasing average costs for any conceivable level of demand, the company would be making an increasing returns to scale which means that it would be making more return per capital spent.
This will create a natural monopoly because the company will be more efficient in this particular industry and if another company tried to come in, they would have to spend a lot of money to get to a point of increasing returns to scale.