Answer:
C.
Step-by-step explanation:
The First Amendment of the US Constitution protects the right of individuals' free speech. But there are some exceptions to the freedom of free speech that are not protected by the First Amendment. The exceptions include obscenity, fraud, threats, etc.
The sentence that exemplifies a valid restriction of free speech is in option C. The speech in the given statement entices threat with the use of the word 'fire' as a practical joke. The use of this word may produce a threat at the movie theatre.
Therefore, option C is correct.