The correct answer is option 'A' - Software.

Explanation:
A computer is a combination of hardware and software. Let's understand what hardware and software refer to and how they are essential components of a computer system.

1. Hardware:
Hardware refers to the physical components of a computer system. These are tangible parts that you can touch and see. Examples of computer hardware include the monitor, keyboard, mouse, central processing unit (CPU), memory, hard drive, and other peripherals.

2. Software:
Software, on the other hand, refers to the intangible instructions or programs that tell the hardware what to do. It is a collection of data or instructions that guide the computer system in performing specific tasks. Software can be categorized into two types:

a) System software:
System software is responsible for managing and controlling the computer hardware. It provides a platform for running application software. Examples of system software include operating systems like Windows, macOS, and Linux.

b) Application software:
Application software is designed to perform specific tasks or functions for the user. It helps users accomplish their goals or solve specific problems. Examples of application software include word processors, spreadsheets, web browsers, and multimedia players.

Importance of Software in a Computer System:
Software plays a crucial role in a computer system. Here's why it is important:

1. Enables User Interaction:
Software provides a user-friendly interface that allows users to interact with the computer system and perform various tasks. It enables users to run programs, browse the internet, create documents, play games, and much more.

2. Controls Hardware:
Software acts as a mediator between the user and the computer hardware. It controls and manages the hardware resources, ensuring they are utilized efficiently. For example, the operating system manages memory, disk space, and CPU usage.

3. Extends Functionality:
Software extends the functionality of a computer system. With different software applications, users can perform a wide range of tasks, such as editing documents, creating presentations, editing photos, and playing music or videos.

4. Flexibility and Upgradability:
Software can be easily updated or upgraded to incorporate new features, fix bugs, or enhance performance. This allows users to keep their computer systems up to date and adapt to changing technology requirements.

Conclusion:
In conclusion, while hardware forms the physical components of a computer system, software provides the instructions and programs that enable the hardware to perform specific tasks. The combination of hardware and software is essential for a computer to function effectively.