D. More Americans were now able to move to the suburbs.
Population growth after World War II was a cause of expansion of cities into suburbs. The prices of homes in suburbs were more affordable to middle class families, due to lower land prices and new building practices like tract housing. With the growth of the suburbs, improvement of roadways became a priority. The Federal-Aid Highway Act passed in 1956 advanced and enabled further growth of the suburbs surrounding city centers.
The answer to this question is the letter "D" which is "More Americans were now able to move to the Suburbs". The suburbs are the residential part of the district and it is outlying. This is the best result of the creation of the interstate highway system focused and budgeted by the government to improve the means of transporting and moving in different part or areas of the region.
