Which statement regarding sleeper agents in the United States is most accurate?

The statement that sleeper agents' existence has proven to be a real possibility is accurate because there have been documented instances where individuals have been planted by foreign intelligence services to live undetected within the United States, appearing as ordinary citizens while gathering intelligence or preparing for potential operational activities. This concept is not merely theoretical; there have been cases in history where sleeper agents have been activated for clandestine missions, highlighting the genuine threat they pose. Recognizing the potential for sleeper agents informs national security strategies and intelligence operations aimed at prevention and detection. Addressing this threat is crucial for maintaining security and protecting against espionage or terrorism.