The correct answer is “Protected tariffs”
The American System was an economic plan that was written by Henry Clay and other politicians - John C Calhoun and John Quincy Adams - and aimed to strengthen and to unify the American Nation.
The plan consisted of many policies, such as tariffs to protect America’s industries, establish a national bank with a single currency that made trade easier and also improve the country’s infrastructure.
Those tariffs would raise funds so internal improvements could be made, one of them would be the transportation networks that would link the Midwest and North.