Any storage device added to a computer beyond the immediately usable main storage is known as

ANS:C - Backing store

The term backing storage refers to any non-volatile data storage that will retain a computer's data, even after the computer is powered off. Common types of backing storage devices are hard drives, SSD, external hard disk drives, optical media such as CD or DVD, and flash media such as thumb drives and memory sticks. Older computer systems also used floppy disks and magnetic tapes as backing storage. On modern systems, especially computers like netbooks, cloud storage services may also be used as backing storage.

Why is backing storage needed?

Without some type of backing storage, the computer would not have access to its data after the computer is turned off. For example, volatile memory like RAM loses all information if the computer is turned off or rebooted. To prevent information you have been working on from being lost, it is saved to a backing storage device like a hard drive.

So, the option hard disk and floppy disks are under backing storage type, option C is right.