The CPU, or Central Processing Unit, is the hardware component responsible for processing information and executing instructions in a computer system. It is often referred to as the "brain" of the computer, as it performs all the necessary calculations and operations required for the system to function. The motherboard is a circuit board that connects various components of the computer, including the CPU, but it does not directly process information. The hard drive, on the other hand, is a storage device used for long-term data storage, and it does not handle processing instructions. Therefore, the correct answer is the CPU.