The correct answer is 'C. Floppy disk' because floppy disks have a slower data access speed compared to other storage devices such as hard drives or solid-state drives. This is due to the physical limitations of floppy disks, which use a spinning magnetic disk and read/write head to access data, resulting in slower read and write speeds. Therefore, when considering data access speed, floppy disks are the slowest option among the choices provided.