You have just finished replacing the motherboard in your friend Ethan's computer with the same type of motherboard it originally had. You tested the motherboard by powering it on and verifying that the operating system started, and his applications were working. Now Ethan is complaining that his computer no longer recognizes his external hard drive.
Which of the following is MOST likely causing the external hard drive not to function?
A. The USB front panel connector was not properly connected.
B. The computer's power supply was not connected properly.
C. Ethan's hard drive has failed.
D. The replacement motherboard does not support external hard drives.

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.