Explanation:

When the CPU needs to transfer data to or from main memory, it uses specific registers for this purpose: * **Memory Address Register (MAR):** This register holds the address of the memory location that the CPU wants to access (either to read from or write to). * **Memory Data Register (MDR):** This register holds the data that is being transferred to or from the memory location specified by the MAR. If the CPU is writing to memory, the data is placed in the MDR. If the CPU is reading from memory, the data fetched from memory is placed in the MDR.\nTherefore, MAR and MDR are the two registers primarily used for data transfer between the CPU (processor) and memory. (a) IR (Instruction Register) holds the current instruction. (b) PC (Program Counter) holds the address of the next instruction. (d) IR (Instruction Register) is not for data transfer between processor and memory.