Which development was most important in encouraging the growth of the cattle industry during the late 1800s?

The correct answer is:

Expansion of railway lines.

The cattle industry got to its peak between 1867 and the early 1880s, partially due to the increased number of railway lines able to transport cattle to new markets. The rail transport not only changed the velocity of transportation but as tracks were set and refrigerated rail vehicles were produced, trains could go to where the cattle were located.