The correct answer to this open question is the following.
Although there are no options attached we can say the following.
Unfortunately, you forgot to mention what is the "period" you are referring to. At what time in US history you are talking about? Without that period in time, it could be any time in history.
Trying to help, we are going to assume that you are referring to the 20th century.
During the middle to end of the 1900s, the states witnessed the advent of many technological and scientific innovations in infrastructure that benefit the growth and prosperity of the country. Better roads, better means of transportation, modern technology in the industries, allowed the states to expand their economic power because their people had the money to buy goods, and companies were able to create more jobs, allowing the economy to circulate and improving the living conditions of people in each state.
During this time the federal government create the proper legislation to allow economic growth, public and private investment, and just intervened in the economy when it was necessary.