Growth of the railroads.The government gave the railroad companies huge amounts of land for every mile of track laid. This land was used to develop new towns and cities, greatly reducing the area of free pasturage available to cattle owners for their herd.
Increasing numbers of new immigrants. Many of these European immigrants set up as sheep farmers, which required much less capital investment than cattle farming. The sheep farmers fenced off their land, further reducing available free pasturage for cattle, and additionally denying access to vital water sources for the cattle herds.
I'm pretty sure (1.) is A, and I know for a fact that (3.) is D because slavery was a regular practice between quarreling tribes. I have no clue what (2.) is. Best of luck.
