The correct answer is A. Hardware.
Hardware is the physical components of a computer system. It includes the CPU, memory, storage, input/output devices, and any other physical parts of the system.
Software is the set of instructions that tells the hardware what to do. It includes the operating system, applications, and any other programs that are installed on the system.
Input/output (I/O) devices are the hardware components that allow the user to interact with the computer. They include the keyboard, mouse, monitor, printer, and any other devices that are used to input or output data.
None of these is the correct answer because it does not describe the physical components of a computer system.