The physical parts of a computer system are called:

A computer system comprises both tangible and intangible components. The question asks for the term describing the physical parts.

• Hardware is the correct answer. Hardware refers to all the physical, tangible components of a computer system that you can see and touch, such as the CPU, memory, hard drive, keyboard, and monitor. These are the electronic and mechanical parts that make up the computer.
• Software (B) refers to the intangible set of instructions, programs, and data that tell the hardware what to do. It cannot be physically touched.
• Firmware (C) is a specific type of software embedded into hardware devices, providing low-level control, like the BIOS. While it's software, it's not the overarching term for all physical parts.
• Operating System (D) is a type of software that manages computer hardware and software resources, providing common services for computer programs. It is a crucial piece of software, not a physical part itself.

Therefore, 'Hardware' is the most accurate term for the physical components.