Answer:
D. of widespread entrepreneurship.
Step-by-step explanation:
Competitive economies are not characterized by imposing restrictions on foreign trade (option B) or necessarily saving more than anyone else (option A). Even the amount of hard working citizens is not a real measure, in some Asian countries people work for 16 hours per day and earn just a few a cents (option C). What makes an economy competitive is how many new innovations and how many new businesses appear every year. They don't even have to be made by Americans, but people living in America. E.g. Apple, Google, Tesla and many others were built by immigrants that found the opportunity to succeed in America.