The "brain" of the computer is the:

The central processing unit (CPU) is referred to as the "brain" of the computer because it is responsible for executing instructions and processing data. The CPU carries out the various computations and operations required to perform tasks as dictated by the software and programmable logic. It interprets and carries out commands from the operating system and applications, making it integral to overall system functionality.

In addition to being the main processing unit, the CPU also communicates with other components, coordinating activities among the memory, input/output devices, and storage. This central role in controlling and managing data processing activities reinforces its designation as the brain of the computer.

In contrast, other components such as programmable read-only memory (PROM) and random access memory (RAM) serve specific functions; PROM is used for permanent storage of firmware, while RAM provides temporary storage for data and instructions that are actively being processed. The analog-to-digital (AD) converter is a device that converts analog signals into a digital format but does not perform processing in the same capacity as the CPU. Thus, while each of these components plays a vital role in the computer system, none replace the essential data processing and control functions performed by the CPU.