BIOS is used by:

The correct answer is Microprocessor  

Key Points

• The BIOS (Basic Input/Output System) is a firmware used by the microprocessor during the boot process to:
• Initialize and test hardware components (POST - Power-On Self-Test).
• Load the bootloader, which then starts the operating system.
• Provide low-level hardware control to the OS

Additional Information 

• The operating system (OS) manages all of the software and hardware on the computer. It performs basic tasks such as file, memory and process management, handling input and output, and controlling peripheral devices such as disk drives and printers.
• A piece of equipment/hardware which gives out the result of the entered input, once it is processed, is called an output device.
• A microprocessor is any type of miniature electronic device that contains the arithmetic, logic, and control circuitry necessary to perform the functions of a digital computer's central processing unit.
• Hardware refers to the external and internal devices and equipment that enable you to perform major functions such as input, output, storage, communication, processing, and more.