The most accurate response to why McCarthyism was able to spread so easily in the United States is:
The Rosenberg trial led the public to believe there were more communists in the country.
This response highlights how the highly publicized trial and execution of Julius and Ethel Rosenberg for espionage heightened fears of communism and contributed to a climate of suspicion and paranoia, making it easier for McCarthyism to take hold in American society.