Answer: The freedom of speech protections in the First Amendment to the U.S. Constitution
Explanation: New York Times Co. v. Sullivan (1964), was a landmark decision of the U.S. Supreme Court in which the Court ruled that the freedom of speech protections in the First Amendment to the U.S. Constitution restrict the ability of American public officials to sue for defamation. The Supreme court specifically held that if a plaintiff in a defamation lawsuit is a public official or person running for public office, not only must they prove the normal elements of defamation, they must also prove that the statement was made with "actual malice", meaning that the defendant either knew the statement was false or recklessly disregarded whether or not it was true.
The Court said it was necessary to protect the erroneous statements about public officials that would come up in free debate, otherwise critics of public officials will censor their speech for fear of potentially unlimited liability which could severely limit the information that could be given or published about the character of such officials.