Which of the following actions can result in a warrantless arrest?

A warrantless arrest can occur under certain circumstances when probable cause exists that a crime has been committed. Selling alcohol without a license is a violation of the law, and law enforcement officers have grounds to believe that a crime is taking place if they observe this activity directly. This allows them to make a warrantless arrest based on the immediate evidence of illegal selling.

In contrast, engaging in a peaceful protest is generally protected under the First Amendment, and individuals cannot be arrested simply for exercising this right, unless there is a violation of the law that accompanies the protest. Conducting a legal search is not a basis for warrantless arrest; rather, it is an expression of lawful police conduct, which may involve obtaining a warrant. Walking on a public sidewalk is also not a criminal act, so it does not provide grounds for a warrantless arrest. Therefore, selling alcohol without a license is the correct choice because it involves a clear violation of the law that justifies immediate action by law enforcement.