Which command is used to check the memory usage of a device?

The command used to check the memory usage of a device is "display memory-usage." This command directly provides information regarding the amount of memory being used by the device, offering insights into system performance and resource allocation. It is crucial for network administrators to monitor memory usage, as excessive memory consumption can lead to degraded performance or even system failures.

The other commands listed serve different purposes: "display device" shows general information about the device's configuration and status, "display fan" presents the operational status of the device's cooling fans, and "display running-status" provides an overview of the device's current operational status without specifically detailing memory usage. Therefore, "display memory-usage" is the specialized command designed to target memory metrics specifically.