What type of pluggable devices can only be connected when the system is powered down?

The term that describes devices which can only be connected when the system is powered down is "cold pluggable." Cold pluggable devices require a complete shutdown of the system before they can be added or removed. This is primarily due to the potential risks associated with electrical connections or disconnections while the system is operating, which could cause hardware damage or system instability.

In contrast, hot pluggable devices can be connected or disconnected while the system is running, allowing for greater flexibility and convenience. Dynamic and static pluggable do not accurately describe a standard category of device connection in this context. Dynamic generally refers to changes that can occur in a system while it is operating, while static often implies a fixed state without alteration. Therefore, cold pluggability directly refers to the necessity of powering down the system for connecting certain types of devices, making it the appropriate choice in this scenario.