"c. social welfare programs" and "<span>e. communist spies in the United States" are the best options, since these two events brought the possibility of communism to the homeland. </span>
I believe it is c i looked it up and my 8th grade teacher from bout 3 years ago said it i think
<span>The United States amended the Constitution to make communism illegal.</span>
Other words that go with the same meaning of common knowledge include Ongoing, existence, widespread, popular, etc.