What issue might cause a newly installed processor not to show up during POST?

A newly installed processor might not show up during the POST (Power-On Self-Test) if the voltage regulator module (VRM) was not installed. The VRM is responsible for supplying the correct voltage to the processor, which is crucial for its operation. If the VRM is absent or malfunctioning, the processor may not receive the necessary power levels to initialize, preventing it from being recognized by the system.

The other options, while potentially impactful in different contexts, are less directly related to the fundamental recognition of a processor during POST. Insufficient power supply can lead to broader system failures or instability, but it would generally affect multiple components rather than specifically prevent the processor from being acknowledged. IRQ and DMA settings, on the other hand, relate to how devices communicate with the CPU and are more relevant after the CPU is recognized and initialized rather than during the initial boot process. Therefore, the failure to have the VRM installed directly addresses the issue of processor acknowledgment during POST.