Which of the following is considered a barrier in security?

A locked door is indeed a physical barrier in security because it provides a tangible means of restricting access to a specific area. When a door is locked, it prevents unauthorized individuals from entering, thus safeguarding the contents or people within. This physical barrier is one of the most basic yet effective forms of security, as it requires a key or access control to open, effectively limiting access to those who are supposed to be in that area.

In contrast, while a person guarding an entrance, an alarm system, and a surveillance camera all play crucial roles in a security strategy, they do not serve as physical barriers in the same way a locked door does. The presence of a guard can deter unauthorized entry but does not physically prevent it. An alarm system can alert to breaches but does not stop someone from entering. Surveillance cameras monitor activity and can provide evidence, but they also do not stop an intruder. Thus, a locked door stands out as a definitive barrier that actively restricts entry.