Registers are the fastest among the given options.

Registers:
- Registers are small, high-speed storage locations that are part of the CPU.
- They hold data that the CPU is currently processing or about to process.
- Registers are used to store data that is frequently accessed by the CPU, such as data used in arithmetic operations.
- Registers are also used to hold memory addresses, instruction pointers, and other important CPU information.

CD-ROM:
- CD-ROM stands for Compact Disc Read-Only Memory.
- It is a type of optical disc used for storage.
- Data is read from a CD-ROM using a laser beam.
- CD-ROMs are relatively slow compared to other storage devices, such as hard drives or solid-state drives.

RAM:
- RAM stands for Random Access Memory.
- It is a type of computer memory that stores data temporarily.
- RAM is used by the CPU to store data that is currently being processed.
- RAM is faster than most storage devices, but slower than registers.

Cache:
- Cache is a type of memory used to store frequently accessed data.
- It is faster than most storage devices, but slower than registers.
- Cache is used to improve the performance of a computer by reducing the number of times data needs to be accessed from slower storage devices, such as hard drives or RAM.

In conclusion, registers are the fastest among the given options because they are part of the CPU and hold data that is currently being processed. They are faster than RAM and cache because they are smaller and have a shorter access time. CD-ROMs are relatively slow compared to other storage devices and are not used for processing data.