what would have been most prohibitive to homesteaders wishing to travel by Steamboat

The most prohibitive aspect of steamboat travel for homesteaders would have been the cost.  

Explanation:

For homesteaders who were looking to move west to regions like Montana in the 1880s, steamboats were an option but there were a number of obstacles or challenges with taking a steamboat.  First and foremost was likely the price as the fare was about $300, and cargo would cost the families about 12 cents a pound. This was expensive for average families looking to establish themselves as farmers once they reached their destinations. It was also a more dangerous journey than is typically depicted. The Missouri River was really only navigable for a short window in the summer months, and boats frequently ran into problems running aground and the boilers might overheat and explode as a result.

answer; the correct answer is (travel horizontally);