Final answer:
The Authorizing Official or a designated Security Control Assessor is responsible for determining and documenting the risk level of every NIST security control in the SAR for federal information systems.
Step-by-step explanation:
The individual who determines and documents the risk level for every National Institute of Standards and Technology (NIST) security control in the system baseline in the Security Assessment Report (SAR) is typically the Authorizing Official (AO) or an appointed representative such as a Security Control Assessor (SCA). The NIST framework is commonly used in the USA for federal information systems to ensure they meet specific security requirements. The assessment process involves evaluating each security control, documenting the findings, determining risk levels, and ultimately recommending whether the system should receive authorization to operate based on the identified risks.