The correct answer is C. CPU.
The CPU is the brain of the computer. It is responsible for carrying out the instructions of the computer program. It does this by performing arithmetic and logical operations on data. The CPU is also responsible for controlling the other parts of the computer, such as the memory and the input/output devices.
The ALU is the arithmetic and logic unit. It is responsible for performing arithmetic and logical operations on data. The ALU is not the brain of the computer, because it does not control the other parts of the computer.
The input device is the part of the computer that allows the user to input data into the computer. The input device is not the brain of the computer, because it does not control the other parts of the computer.
The control unit is the part of the computer that controls the other parts of the computer. The control unit is not the brain of the computer, because it does not perform arithmetic and logical operations on data.
Therefore, the correct answer is C. CPU.