Choice "D" is correct. Large capital (money) requirements are the basic example of barriers to entry. A barrier to entry effectively prevents firms from entering the market to compete against existing firms.
Choice "c" is incorrect. Minimum efficient scale is the output level at which long run average costs are minimized. Here, Parker has not even been able to enter the industry.
Choice "b" is incorrect. A created barrier is made by firms already in the industry. Here, Parker's barrier was not created.
Choice "a" is incorrect. An external cost is a cost that the company does not account for, but passes on to the detriment of society.