Which of the following is a Fourth Amendment violation?

A Fourth Amendment violation occurs when a law enforcement officer conducts an unreasonable search or seizure. In this context, the act of arresting someone without probable cause is a clear infringement on the Fourth Amendment rights. Probable cause is a legal standard that requires law enforcement to have a reasonable basis for believing that a crime has been committed or will be committed. If an officer makes an arrest without this justification, it is considered unlawful and violates the individual's constitutional protections against unreasonable search and seizure.

The other options do not constitute Fourth Amendment violations under normal circumstances. Requesting consent for a search from someone who is willing indicates that the search is conducted with the individual's agreement, which is permissible. Conducting a stop and frisk based on reasonable suspicion aligns with established legal standards that allow police to briefly detain a person if they have specific and articulable facts suggesting criminal activity is afoot. Lastly, the selection of individuals for searches based on behavior is also permissible as long as it is based on observable, specific, and reasonable indicators of potential criminal activity, thereby not breaching the Fourth Amendment.