Under which circumstance may the government restrict freedom of speech?

The correct answer highlights a fundamental principle in constitutional law regarding the limits of free speech. The government is permitted to restrict freedom of speech when that speech incites violence or poses a clear and present danger to society. This principle stems from the recognition that certain expressions can lead to harmful consequences that threaten public safety or order.

The "clear and present danger" test, established in the landmark case Schenck v. United States (1919), indicates that speech can be limited if it is likely to produce imminent lawless action or if it poses an immediate threat. For instance, inciting a riot or encouraging violence against specific individuals or groups falls under this category. This ensures that while free speech is a protected right, it does not extend to actions that could directly endanger others or disrupt societal peace.

In contrast, the other scenarios align with sentiments that do not meet this threshold. Public opinion or unpopularity of speech does not provide sufficient grounds for restriction, as the First Amendment protects even controversial or unpopular viewpoints. Similarly, acts deemed necessary for national security must still be narrowly defined and not violate fundamental rights without just cause. Therefore, it's crucial to understand that speech is only subject to restriction when it poses a legitimate threat of harm or violence.