Explanation:

- The term "brain of the computer" refers to the component that performs the majority of processing tasks.

- Option 1: CPU

- The CPU is the Central Processing Unit.

  It processes data and executes instructions.

- It is often called the brain of the computer.

- Option 2: Motherboard

- The motherboard is the main circuit board.

- It connects all components including the CPU, memory, and peripheral devices.

- It is not the brain but more like the "central nervous system."

- Option 3: HDD

- The HDD stands for Hard Disk Drive.

- It is used for storage, not processing.

- It stores data, files, and the operating system.

- Option 4: Hardware

- Hardware refers to all physical components of a computer.

- It includes the CPU, motherboard, HDD, and more.

The correct answer is Option 1: CPU.