A security guard opens the door to allow an authenticated person into the data center. A second person enters behind the first person without properly scanning through the security. This method of circumventing physical access controls is called

The correct answer is piggybacking, or tailgating. This term specifically describes the scenario where an unauthorized individual gains access to a secure area by following closely behind an authorized individual who has legitimately authenticated their entry. This technique exploits the trust and social norms associated with physical security, where the first individual may unknowingly allow the second person to gain access without proper verification.

Understanding this concept is essential, as it highlights the importance of not only implementing strict access control measures but also ensuring that individuals are trained and aware of the tactics that can be used to bypass these measures. This includes being vigilant about who is entering secure areas and ensuring that only individuals who have been verified are allowed access.

In contrast, the other options present different concepts. The access control vestibule refers to a secured area designed to limit access and ensure proper authentication but does not specifically address the act of following someone in without authorization. A backup plan relates to contingency strategies in case of unexpected events, which is not relevant to the access issue at hand. Unlawful access is a broader term that describes unauthorized entry without specifying the method, making it less precise than the term piggybacking or tailgating.