Final answer:
The issue with the hard disk light not turning on is likely due to the hard disk LED cable not being connected to the motherboard; hence, you should connect the hard disk LED to the motherboard.
Step-by-step explanation:
If you have just replaced the motherboard in your computer and the hard disk light does not come on while the system is booting, you should not immediately assume there is an issue with the hard disk itself. The key piece of information here is that the hard disk light is not turning on.
When the system boots, this light typically indicates hard disk activity. If everything else appears to be functioning properly, but the light doesn't turn on, it is likely a case of a small oversight: the hard disk LED cable might not be connected to the motherboard. Therefore, the answer is D. Connect the hard disk LED to the motherboard.