Final answer:
The category best described by the statement "The system is protected against unauthorized access" from the AICPA Trust Services Principles is 'Security.' This principle ensures the protection of systems against unauthorized incursions.
Step-by-step explanation:
The AICPA Trust Services Principles outline five categories of IT risks and controls to help organizations manage the security, availability, processing integrity, confidentiality, and privacy of their systems. The statement "The system is protected against unauthorized access" pertains to the category of security within these principles. This category specifically deals with protecting information, as well as the systems that manage this information, against unauthorized access, disclosure, alteration, destruction, and other risks that could potentially compromise the information's security.