The main component of a computer system that performs calculations and makes decisions is:

The CPU (Central Processing Unit) is often referred to as the 'brain' of the computer. It is responsible for executing instructions, performing arithmetic and logical operations, and managing the overall flow of data and instructions within the computer system. Without a CPU, a computer cannot function.

• (A) CPU is correct because it is the primary component for calculations and decision-making.
• (B) RAM (Random Access Memory) is incorrect; RAM is used for temporary data storage, not processing.
• (C) Monitor is incorrect; a monitor is an output device that displays visual information.
• (D) Hard Drive is incorrect; a hard drive is a storage device for long-term data retention.