The correct answer is A. Semiconductor memory.
Semiconductor memory is the fastest type of memory, and it is used for storing data that needs to be accessed quickly, such as the operating system and applications. Semiconductor memory is made up of transistors, which are tiny switches that can be turned on or off to represent data.
Magnetic disks are a type of secondary storage that is used to store large amounts of data. Magnetic disks are made up of a thin metal disk that is coated with a magnetic material. Data is stored on the disk by magnetizing tiny areas on the surface of the disk.
Magnetic drums are a type of secondary storage that is similar to magnetic disks. However, magnetic drums are cylindrical in shape, and they rotate much faster than magnetic disks. This makes magnetic drums faster than magnetic disks, but it also makes them more expensive.
Magnetic tapes are a type of secondary storage that is used to store large amounts of data. Magnetic tapes are made up of a thin plastic tape that is coated with a magnetic material. Data is stored on the tape by magnetizing tiny areas on the surface of the tape.
Magnetic tapes are the slowest type of memory, and they are used for storing data that does not need to be accessed quickly, such as backup data.