Final answer:
Main memory is the device used during the processing stage of computing, as it holds data and programs for the CPU to process.
Step-by-step explanation:
Main memory is the device used during the processing stage of computing, as it holds data and programs for the CPU to process.The correct device used during the processing stage of computing is B) main memory. During the processing stage, the computer's main memory is actively engaged as it holds the data and program that are currently being processed by the CPU (Central Processing Unit).
The UPC reader and bar code scanner are input devices, which capture data from the external world to be processed, while the printer is an output device that brings digital information into the physical world as printed material. It is the main memory that interacts with the CPU during the crucial stage when the computer actually processes data, following the input stage and preceding the output stage.