What is the most likely reason a 64-bit OS cannot be installed on a server?

The correct answer is that the server has the wrong architecture. A 64-bit operating system requires a compatible 64-bit architecture to function properly. This means that the server’s CPU must be designed to support 64-bit instructions. If the underlying architecture is 32-bit, a 64-bit OS simply cannot be installed since the hardware cannot adequately process the larger data widths and specific instructions designed for 64-bit operation.

Architecture refers not only to the CPU itself but also includes the motherboard and the firmware. If the server was built on a foundation that is fundamentally only capable of 32-bit processing, then the installation of a 64-bit operating system is unfeasible, regardless of other factors like processor count or memory size.

While factors such as the number of processors or the amount of memory may indeed affect overall system performance, their compliance with 64-bit OS requirements is secondary to the CPU architecture. If the architecture does not support 64-bit instruction sets, the OS installation will fail irrespective of these other components being adequate.