Explanation:

- CD-ROM, or Compact Disc Read-Only Memory, is a type of data storage medium but not part of a computer's main memory hierarchy.

- Option 1: Secondary memory - Correct. CD-ROM is classified as secondary memory since it stores data externally and non-volatilely for long-term access. It's used for distributing software, games, and other large media files.

- Option 2: Magnetic memory - Involves storage technology like hard drives, using magnetic fields to record data.

- Option 3: Memory register - Found within the CPU, used for processing temporary data at high speeds.

- Option 4: Semiconductor memory - Includes RAM and flash memory, not optical discs like CD-ROM.