Which USB device can be plugged into a switch or router to add storage?

The correct answer is the USB flash drive because it directly serves the purpose of providing additional storage. When plugged into a switch or router that supports USB storage, a flash drive can be used for various functions, such as storing configuration files, backups, or log files. Many network devices allow administrators to save operational data easily by utilizing this method.

In contrast, a USB mouse and a USB keyboard are input devices that facilitate user interaction with the system but do not contribute to storage. Similarly, a USB printer serves a different purpose, primarily for outputting documents rather than storing data. Therefore, while all the choices are USB devices, only the USB flash drive is intended to expand storage capabilities on network equipment like switches or routers.