The correct answer is: A. Power supply.
An ESD (Electrostatic Discharge) is a sudden discharge of static electricity. It can cause damage to electronic components, especially those that are sensitive to voltage spikes. The power supply is one of the most important components in a computer, and it is also one of the most sensitive to ESD. If an ESD event occurs, it can cause the power supply to fail, which can damage other components in the computer.
The other options are also susceptible to ESD damage, but they are not as sensitive as the power supply. A monitor is a display device, and it is not as likely to be damaged by ESD as a power supply. A keyboard is an input device, and it is also not as likely to be damaged by ESD as a power supply. An expansion board is a circuit board that is added to a computer to provide additional functionality. It is also not as likely to be damaged by ESD as a power supply.
In conclusion, the power supply is the component in a computer that is most likely to be damaged by ESD.