Which of the following is NOT an I/O device of a computer?

The central processing unit (CPU) is the primary component of a computer responsible for executing instructions and processing data. It serves as the "brain" of the computer, managing and coordinating all operations. Input/output (I/O) devices, on the other hand, are peripherals that facilitate communication between the user and the computer system. Input devices like a mouse allow users to provide commands or data to the computer, while output devices like a printer or video monitor relay information from the computer to the user.

Therefore, in the context of this question, the CPU does not fit into the category of I/O devices since it is not involved in the direct input or output of information but rather in processing it.