The correct answer is: A. Magnetic drum.
A magnetic drum is a type of primary storage device that uses a rotating drum coated with magnetic material to store data. Data is stored on the drum in the form of magnetized spots, which can be read and written by read/write heads. Magnetic drums are a type of direct-access storage device (DASD), which means that data can be accessed directly without having to search through a linear sequence of data. This makes magnetic drums a fast and efficient storage medium.
PROM (programmable read-only memory) is a type of non-volatile memory that can be programmed once and then read many times. PROM is used to store firmware, which is the software that controls the hardware of a device. PROM is not a primary storage device because it cannot be erased and rewritten.
Floppy disk is a type of magnetic storage medium that was once widely used to store data. Floppy disks are small, portable, and relatively inexpensive. However, floppy disks have been largely replaced by other storage media, such as USB drives and hard drives.
Therefore, the correct answer is A. Magnetic drum.