What is one of the two required elements for a lawful pat down or frisk?

A lawful pat down or frisk is primarily justified when the officer has a reasonable belief that the person being detained may be armed and dangerous. This situation arises during a lawful detention, which is the critical element referenced in the correct choice. The requirement that the person is lawfully detained means that there must be a legitimate reason for the officer to stop and question the individual, such as suspicion of criminal activity.

In this context, the pat down is a limited search conducted for the officer's safety, intended to uncover weapons without infringing too much on the individual’s rights. The law seeks to balance the need for police safety with the protection of individual civil liberties, thereby establishing that a legal basis for the detention must exist before a frisk can be deemed lawful.

Probable cause is a higher standard of evidence and is not necessary for a pat down, which relies on a lower threshold of reasonable suspicion. The other options, such as a suspect being in a car or the necessity of consent, do not relate directly to the authority granted to officers during a lawful stop to perform a frisk for weapons. Hence, being lawfully detained is a key prerequisite that supports the action of conducting a pat down.