A national intelligence agency is designing a new system to process both Top Secret and Unclassified data simultaneously. Engineers propose using a formally verified microkernel operating system that enforces strict separation between processes through hardware-based memory isolation. During review, an executive asks why the team insists on this complex design instead of using simpler software-based access controls at the application layer. Which concept BEST justifies the microkernel approach? A. Complete mediation — ensuring every access request is validated against the security policy. B. Security kernel — implementing reference monitor functions at the lowest level of the system. C. Layered defense — using multiple, independent safeguards at different levels of abstraction. D. Economy of mechanism — minimizing system complexity to reduce potential vulnerabilities.