The correct answer is: A. ROM BIOS
ROM BIOS (Read-Only Memory Basic Input/Output System) is a small program that is stored on a chip on the motherboard of a computer. It is responsible for initializing the hardware and loading the operating system when the computer is turned on. ROM BIOS is not affected by changes to the operating system, so it is the least likely to interfere with upgrading to a newer DOS version.
The other options are all hardware components that can be affected by changes to the operating system. For example, if the video controller is not compatible with the new DOS version, the computer may not display correctly. Compressed drives may also not work with the new DOS version, and existing backup files may need to be converted to the new format.
Therefore, the correct answer is A.