Answer: A. The USB front panel connector was not properly connected
Explanation:
From the information given, the most likely reason causing the external hard drive not to function is that the USB front panel connector was not properly connected.
Even if the computer's power supply was not connected properly, this won't cause the external drive not to function. Also, the failure of the hard drive wouldn't be an option.
If the USB front panel connector was properly connected, then the computer will recognize his external hard drive.