Which FPCON level is characterized by mandatory personnel-only access?

FPCON Charlie is the correct answer because it is characterized by the highest level of security measures in response to a probable threat. During FPCON Charlie, access to the designated area is restricted to authorized personnel only, ensuring that sensitive locations are protected from potential threats. This heightened security often includes measures such as increased surveillance, access control, and the presence of security forces to monitor and manage who enters the area.

Understanding the context of the other FPCON levels helps clarify why they do not align with the characteristics of FPCON Charlie. For instance, FPCON Alpha involves a general threat of terrorist activity and allows for increased security measures, but does not restrict access to only personnel. FPCON Bravo indicates an increased and more predictable threat, still permitting broader access. Finally, FPCON Delta represents a specific and imminent threat to a facility, which may involve lockdowns and even stricter measures than FPCON Charlie but still transitions from or to specific elevated readiness states rather than merely focusing on personnel-only access. Thus, the defining characteristic of mandatory personnel-only access makes FPCON Charlie the appropriate answer.