The system unit is the main component of a computer system that houses all the essential hardware components required for processing and storing data. It is sometimes referred to as the central processing unit (CPU) or the computer tower. Among the given options, the part of the system unit is:

d) Memory

Memory is an essential component of the system unit and refers to the storage space that the computer uses to temporarily store data, instructions, and information. It is responsible for holding data that is currently being used by the computer's processor.

Memory can be further classified into two types:

1. Random Access Memory (RAM): RAM is the primary memory of the computer and is used to store data and instructions that are currently being processed by the CPU. It is volatile memory, meaning that its contents are lost when the computer is turned off or restarted.

2. Read-Only Memory (ROM): ROM is a type of memory that stores permanent instructions and data that cannot be modified or erased. It contains the firmware or software instructions that are required to boot up the computer and initialize the hardware components.

Memory is crucial for the overall performance of a computer system. Insufficient memory can lead to slow processing speeds and system crashes. It is one of the factors that determine a computer's multitasking capability and ability to run resource-intensive applications smoothly.

In summary, memory is an integral part of the system unit and is responsible for storing and retrieving data and instructions during the computer's operation. It plays a crucial role in determining the computer's performance and multitasking capabilities.