The correct answer is D. power supply.
A power supply is an important component of a computer that provides power to all the other components. If the power supply is not working properly, the computer will not boot.
The other options are not as likely to be the cause of the problem. A hard drive is a storage device, and a video card is a graphics card. If either of these components were not working properly, the computer would still be able to boot. A power cable is a simple cable that connects the power supply to the wall outlet. If the power cable was not working properly, the computer would not turn on at all.
To test if the power supply is the problem, you can try using a different power supply with the computer. If the computer boots with a different power supply, then the original power supply is the problem. You can also try using a power supply tester to test the power supply.
If the power supply is the problem, you will need to replace it. You can purchase a new power supply from a computer store or online. When replacing the power supply, be sure to follow the instructions that come with the power supply.