The correct answer is: D. Extended Memory
HIMEM.SYS is a DOS device driver that allows DOS to access extended memory. It is loaded by the CONFIG.SYS file and must be present in order to use extended memory. HIMEM.SYS does not control DOS=HIGH, RMDRIVE.SYS, or SMARTDRV.SYS.
DOS=HIGH is a command that can be used in the CONFIG.SYS file to load DOS into high memory. This can be useful if you have a computer with a small amount of conventional memory. RMDRIVE.SYS is a device driver that can be used to create virtual drives. SMARTDRV.SYS is a device driver that can be used to improve the performance of your hard drive.
Extended memory is a type of memory that is not accessible to DOS by default. HIMEM.SYS allows DOS to access extended memory, which can be used to run programs that require more memory than is available in conventional memory.