Which of the following will allow devices to be added without turning the server off?

Plug and Play (PnP) is a standard for automatic hardware detection and driver assignment, but it does not by itself define the ability to physically insert hardware while the system is running.

A warm site is a disaster recovery term for a backup facility with partially pre-configured hardware and infrastructure, unrelated to hardware insertion.

Hot plug, also called hot swapping, allows components such as drives, NICs, or PCIe cards to be physically inserted while the server remains powered on and operational. The operating system detects the new hardware in real time and loads the appropriate drivers without requiring a reboot. This feature is critical in enterprise environments where continuous uptime must be maintained.

A hot site is a disaster recovery term referring to a fully operational backup data center that can take over immediately, not a hardware-level feature.