The correct explanation of how selective incorporation contributed to the right to freedom of speech in the United States is:
Selective incorporation required that the right to freedom of speech be protected by state governments.
Selective incorporation is the doctrine used by the Supreme Court to apply certain protections in the Bill of Rights to the states through the Fourteenth Amendment's Due Process Clause. This means that state governments are also required to uphold the right to freedom of speech, not just the federal government.