After a Certifying Authority accredits a system, what formal letter is granted to the system owner, allowing the system to operate for a period of three years?

The correct answer is that after a Certifying Authority accredits a system, the formal letter granted to the system owner is the Authorization to Operate (ATO). This document signifies that the system has been evaluated and deemed compliant with security standards necessary to mitigate potential risks. It allows the system to operate legally within its defined environment for a specified duration, which is typically three years.

The ATO encompasses several critical assessments, such as evaluating the system's security controls and ensuring that they are implemented effectively. It serves as a formal approval, indicating that the system meets all necessary requirements to operate and that any identified security risks have been adequately addressed or accepted by the appropriate authority.

Other options relate to different aspects of the cybersecurity accreditation process. For instance, while certification and accreditation are related concepts, the ATO is the specific letter that grants permission for operation. A Plan of Actions and Milestones (POAM) outlines the plans to resolve identified vulnerabilities but does not grant operating authority. Hence, the ATO plays a pivotal role in establishing confidence in the security of the system while enabling continued operational capability.