Final answer:
Guillermo is acting as a User when he logs onto a system and opens a document file, which grants him permission to access certain resources within allowed limitations.
Step-by-step explanation:
When Guillermo logs onto a system and opens a document file, he is playing the role of a User in terms of access control. A user is someone who has been granted permission to access the system and make use of its resources, such as files and applications, generally within certain limitations set by the administrators who manage the system. The options Administrator, Auditor, and Intruder do not apply in this context as they have distinct roles: an Administrator has elevated privileges to manage and control system settings, an Auditor typically reviews system access and usage for compliance, and an Intruder is someone who gains unauthorized access to the system, which does not describe Guillermo's legitimate activity.