The correct answer is C. Disk storage unit.
A disk storage unit is a random-access data storage device that gives rapid access to data. It is a type of secondary storage device that uses magnetic disks to store data. The data is stored on the disk in the form of magnetic patterns. The disk is rotated at high speed, and a read/write head is used to access the data. The read/write head can be moved to any location on the disk, which allows for random access to data.
Magnetic tape is a sequential-access data storage device. This means that data can only be accessed in a linear fashion, from beginning to end. Punched cards are also sequential-access data storage devices. Data is stored on punched cards in the form of holes. The holes are read by a machine, which interprets them as data.
A buffer is a temporary storage area that is used to hold data. It is not a data storage device. Data is stored in the buffer while it is being processed. Once the data has been processed, it is removed from the buffer.
Therefore, the correct answer is C. Disk storage unit.