Final answer:
An officer can lawfully initiate a frisk only when they have reasonable suspicion that the person is armed and dangerous, as defined by the Terry v. Ohio case and the Fourth Amendment protections against unreasonable searches and seizures.
Step-by-step explanation:
A lawful frisk can only be initiated by an officer when they have reasonable suspicion that the person being frisked is armed and dangerous. The landmark case Terry v. Ohio established that police may stop and frisk an individual if they possess reasonable suspicion that a person has committed or is about to commit a crime, and this extends to the suspicion that the person is armed and dangerous. Searches and seizures, including frisks, must align with the Fourth Amendment, which requires that the people be 'secure in their persons, houses, papers, and effects, against unreasonable searches and seizures' unless there is probable cause, supported by oath or affirmation, particularly describing the place to be searched, and the persons or things to be seized. In practice, while warrants are generally needed for searches and seizures, there are certain exceptions where officers can act without a warrant, such as with consent, exigent circumstances, or if an item is in plain view.
Concerning the choices presented in the question: (1) A search warrant alone does not justify a frisk; (2) is the correct condition for a lawful frisk; (3) probable cause relates more to arrests and searches rather than to a frisk; and (4) a routine traffic stop does not automatically give rise to a lawful frisk unless there is a reasonable suspicion of danger.