The correct option is A The Pacific Coast and remains
The use of the conjunction 'and' to combine both the sentences is the best choice because 'and' is used to "used to connect two clauses, the second of which refers to something that results from the first." Here, because the Pony Express played an important role in the development of the Pacific Coast, it remains an icon of the Wild West. The options A, B and C are incorrect because they do not serve the same purpose as D does. D is the correct answer.