Domain 4 with the Implement RMF step is correct because this pairing describes the activities that put selected security controls into operation and handle deployment issues in the live environment.

The RMF Implement step follows the selection of controls and concentrates on installing, configuring, and integrating the chosen controls so they function as intended in the operational system. This step also addresses real world deployment problems such as compatibility, tuning, and operational constraints.

Domain 4 in governance risk and compliance frameworks generally maps to the practical implementation and operationalization of controls. That mapping makes Domain 4 the best fit for the RMF Implement step when the focus is on bringing controls into service and resolving deployment issues.

Domain 6 with the Monitor RMF step is incorrect. The Monitor step concerns ongoing oversight and continuous assessment of controls after they are in place rather than the initial act of putting controls into operation.

Domain 2 with the Categorize RMF step is incorrect. The Categorize step is about defining the system, its boundaries, and the impact levels, and it does not address deploying or operationalizing controls.

Domain 7 with the Integrate RMF step is incorrect. The RMF does not use a formal Integrate phase as named in NIST guidance and domain seven does not specifically describe the operational deployment and configuration activities covered by the Implement step.

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