Question

select the best evidence to support the statement that in the late 1800s, a law made it easier for magazine companies to do business.
at the turn of the twentieth century, short stories were a preferred form of entertainment in the united states. this was a boom time for magazine publishing, owing in part to developments in offset printing technology as well as to the postal act of 1879, which had granted magazines discounted mailing rates. publications such as ladies home journal, the saturday evening post, and the delineator, all of which published short stories, sold more than a half million copies per issue. the authors of these stories were well known at the time and often well paid.
from heidi pitlor, 100 years of the best american short stories. copyright 2015 by houghton mifflin harcourt publishing company

Explanation:

The relevant sentence is "owing in part to developments in offset printing technology as well as the Postal Act of 1879, which had granted magazines discounted mailing rates". The Postal Act of 1879 giving discounted mailing rates is the law that made it easier for magazine companies to do business, as it reduced their mailing costs. Other parts talk about short stories' popularity or printing tech (not the law - related evidence).

Answer:

The sentence: "the Postal Act of 1879, which had granted magazines discounted mailing rates"