Which of the following is NOT a function of a proxy server?

The correct answer indicates that serving as a backup storage is not a function of a proxy server. Proxy servers primarily focus on managing network traffic between clients and the internet, which includes enhancing security by hiding the client's IP address, controlling internet usage by implementing policies on web access, and filtering content to block unwanted or harmful websites.

Backup storage, on the other hand, relates to data preservation strategies where copies of data are made and stored separately to prevent data loss. This function is typically handled by dedicated backup solutions or storage devices rather than by proxy servers. Therefore, understanding the specific roles of a proxy server clarifies why serving as backup storage falls outside its defined functions.