Explanation:

Virtual memory is a memory management technique that allows a computer to compensate for physical memory (RAM) shortages by temporarily transferring data from RAM to disk storage. It creates the illusion of a larger main memory than is physically available, enabling the CPU to run larger programs or multiple programs concurrently by using a portion of the hard disk as an extension of RAM. Option (a) accurately describes this function. Option (b) is incorrect because virtual memory is primarily disk-based, not located in RAM itself. Option (c) is incorrect; virtual memory is used to augment existing RAM, not only when there is no RAM. Option (d) is incorrect as virtual memory is a memory management technique, not a backup device for floppy discs. Hence, (a) is the correct definition.