The building of transcontinental railroads, the discovery of mineral resources, and government policies promoted economic growth and created new communities and centers of commercial activity.
a. Pacific Railway Acts (1862 to 1866)
b. federal and state government subsidies to transcontinental railroads
c. cattle trails
d. cow towns

Option B : federal and state government subsidies to transcontinental railroads

Explanation:

In U.S. History, the building of transcontinental railroads, the discovery of mineral resources, and government policies promoted economic growth and created new communities and centers of commercial activity because larger numbers of migrants moved to the West in search of land and economic opportunity, while the north was still a growing industrial economy, the West still continued to bring in migrants from Mexico as well as from all over the states. In this instance, the new opportunities that the West provided were in the railroad business, new found mineral resources and agricultural possibility, and lastly in the newly provided government policies. The new increase in population increased the number of immigrants too whom had to hunt for their food, so the American Bison population decreased severely.

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i believe the correct answer is b.