A part of Windows 2000 operating system that is not portable is

The correct answer is (B) Virtual Memory Management.

While Windows 2000 was designed for portability using a Hardware Abstraction Layer (HAL) to isolate most hardware-specific code, the Virtual Memory Manager is an exception. It must interact directly with the CPU's Memory Management Unit (MMU), and its design is intrinsically linked to the specific architecture of that hardware (e.g., how page tables and address translation are handled). This deep hardware dependency makes it the least portable component among the choices.