The central processor of a modern digital computer consists of a Control Unit and Primary Memory.

Control Unit:
The control unit is a crucial component of the central processor. It is responsible for executing instructions and coordinating the activities of all other hardware components. The control unit fetches instructions from memory, decodes them, and then directs the other parts of the computer to perform the necessary operations. It controls the flow of data between the various components of the computer, ensuring that instructions are executed in the correct sequence.

Primary Memory:
Primary memory, also known as main memory or random-access memory (RAM), is a hardware component that stores data and instructions that the processor can quickly access. It is volatile memory, meaning that its contents are lost when the computer is powered off. Primary memory is crucial for the functioning of a computer as it holds the instructions and data that are actively being processed by the central processor. It provides fast access to data and instructions, allowing the control unit to retrieve and manipulate them quickly.

Control Unit and Primary Memory:
The control unit and primary memory work together to perform the operations of a digital computer. The control unit fetches instructions from primary memory, decodes them, and then directs the other hardware components to perform the necessary operations. It also stores the intermediate results and final output in primary memory. Primary memory provides the control unit with a fast and efficient means of accessing the instructions and data required for processing.

None of the above (Option D) is incorrect because both the control unit and primary memory are essential components of the central processor. They work in tandem to execute instructions and store data, making them crucial for the proper functioning of a modern digital computer.

In conclusion, the central processor of a modern digital computer consists of a control unit and primary memory. The control unit is responsible for executing instructions, while primary memory provides fast access to data and instructions for processing. Together, they form the core components of the central processor, enabling the computer to perform various tasks efficiently.