Explanation:

The correct answer is option 'B': Main Memory.

Microprocessor and Instructions:
A microprocessor is an electronic component that functions as the central processing unit (CPU) of a computer. It executes instructions and performs calculations required to run programs and perform tasks.

Instructions are the basic operations that a microprocessor carries out. These instructions are stored in memory and retrieved by the microprocessor for execution.

Main Memory:
Main memory, also known as primary memory or random access memory (RAM), is the memory unit that stores data and instructions that are currently being used by the microprocessor. It is a volatile memory, meaning the data is lost when the power is turned off.

Main memory is directly accessible by the microprocessor, and it holds the instructions and data that are needed for the microprocessor's operations. The microprocessor retrieves instructions from the main memory in order to execute them.

Control Memory:
Control memory, also known as control store or control memory unit, is a type of memory that contains microcode instructions. Microcode is a low-level, hardware-specific programming language that directs the microprocessor's operations.

While control memory is used in microprocessors, it is not the primary source from which the microprocessor retrieves instructions. Control memory primarily stores microcode instructions that control the microprocessor's internal operations.

Cache Memory:
Cache memory is a small, high-speed memory unit that stores frequently accessed data and instructions. It is located between the microprocessor and the main memory, acting as a buffer to improve performance.

Cache memory is used to reduce the time it takes for the microprocessor to retrieve instructions and data. It stores a subset of the main memory's content, allowing for faster access. However, the microprocessor does not retrieve instructions directly from the cache memory.

Virtual Memory:
Virtual memory is a memory management technique that allows the execution of programs that are larger than the available physical memory. It uses a combination of main memory and secondary storage, such as a hard disk, to create an illusion of a larger memory space.

While virtual memory is used by operating systems to manage memory, the microprocessor retrieves instructions directly from the main memory and not from the virtual memory.

Conclusion:
In summary, a microprocessor retrieves instructions from the main memory. Main memory is the primary memory unit that holds the instructions and data currently being used by the microprocessor. Control memory stores microcode instructions, cache memory stores frequently accessed data, and virtual memory is a memory management technique. However, the microprocessor retrieves instructions directly from the main memory.