Explanation: When a computer powers on but fails to boot into the operating system, it indicates that the fundamental hardware is receiving power, but the sequence to load the OS is interrupted. **D) Use the BIOS setup mode to check for proper BIOS settings.** The BIOS (Basic Input/Output System) is the first software that runs upon startup. It initializes hardware, performs a Power-On Self-Test (POST), and then looks for a bootable device according to its configuration. Incorrect BIOS settings, such as an improper boot order (e.g., trying to boot from a non-existent drive before the OS drive), a disabled hard drive, or incorrect date/time, can prevent the operating system from loading. Accessing BIOS setup is the most direct first step to diagnose and rectify such issues, as it controls the boot process itself. **A) Make sure the CPU fan is operating normally.** While critical for CPU cooling, a non-operating CPU fan would typically lead to the computer shutting down very quickly due to overheating, or preventing it from even reaching the point of attempting to load an OS. It's not the primary cause of a "powers on but won't boot OS" scenario. **B) Remove the network driver card, clean contacts and reinsert.** A network card issue is unlikely to prevent the entire operating system from booting. If there was a hardware conflict, it's usually much earlier in the POST process or affects specific functions *after* the OS loads. **C) Remove all external cables and try again.** This is a valid troubleshooting step if an external peripheral is causing a conflict or power issue. However, checking the internal BIOS settings is more fundamental to the boot process itself and should be attempted first, as external devices are less likely to prevent the OS from starting altogether than incorrect BIOS configurations. For amateur radio operators, maintaining reliable station computers for logging, digital modes, and station control requires systematic troubleshooting skills. Starting with the most fundamental system configurations, like the BIOS, is key.