Final answer:
To monitor who is accessing files on a Windows server and from which computers, a security analyst should use the 'net session' command.
Step-by-step explanation:
The security analyst who needs to see who is accessing files on a Windows file server and from which computers should use the command E. net session. This command will display all active sessions with the file server from other computers, along with the username and information about each session's status. It's particularly useful for monitoring unauthorized access and managing shared resources.