The correct answer is C.
Schenck v. United States was a US Supreme Court case that took place in 1917 and that was related to the enforcement of the Espionage Act.
The Espionage Act was enacted in order to prevent any interference with military activities or recruitment during war.
The sentence of the Schenck case did set some limits to the right of freedom of speech granted by the First Amendment, in the cases contemplated by the Espionage Act as such speech posed a "clear and present danger" given the war situation. Two activities were handing out leaflets to convince young men from not attending the draft and this obstruction was considered a criminal offence and interfering with military operations.